A 1080
Issue 1, JAN 1965

Advance Information

This Instruction describes the Transmitter Inset No. 16 which is designed for use in the Handset No. 3 and which supersedes the Transmitter Inset No. 13C . It can be used also with Handsets Nos. 4, 5, 6 and 7.

The Transmitter Inset No. 16 is a carbon-granule transmitter with concentric hemispherical carbonised-nickel electrodes. The moving electrode is fixed to a light aluminium diaphragm, this being protected by a perforated front plate which is turned over at the edge to clamp the diaphragm to the body of the transmitter. The back of the transmitter is protected by a black plastic cover in which are moulded two screw terminals for connection of the cord.

There are three types of Transmitter Inset No. 16:-

(a) Mark I. This type has a thin polythene protecting membrane behind the front perforated plate to prevent dust and moisture entering the granule chamber. A black plastic cover is fixed by screws to the alloy body.

(b) Mark II. This is similar to the Mark I except that the alloy body is completely protected by the plastic cover which is clamped by the rim of the front plate.

(c) Mark III. This type has the moving electrode sealed to the diaphragm which is protected by enamel so that the protecting membrane is not required. The plastic cover is fixed to the alloy body by screws as in the Mark I.

Replacement of existing insets
When a Transmitter Inset No. 13C is replaced by a Transmitter Inset No. 16 a spring ring (Part No. 1/DRI/50) must first be fitted in the handset cavity, being located on the three ribs. The transmitter has ribs which should engage in the three channels in the handset cavity, and it is pressed against the inside of the mouthpiece by the spring ring.

When the production rate increases beyond that required for new telephones, the Transmitter Insets No. 16 will be made available for maintenance purposes. An announcement will be made when they are available in Supplies Dept.

Superseded by Transmitter Inset No. 21

New Carbon Transmitter - Transmitter Inset No. 16
A. C. BEADLE, B.Sc.(Eng.), A.M.I.E.E., and F. J. HARVEY, A.M.I.E.R.E.
Taken from the P.O.E.E.J. July 1965

A new transmitter has been developed to supersede the Transmitter Inset No. 13 which has been in service for nearly 30 years. The new transmitter has a slightly improved sensitivity, better frequency response, lower amplitude/amplitude distortion, and freedom from positional effects. Its introduction will complete the development of the 706-type telephone.


THE development of the 700-type telephone circuit envisaged the use of both a new receiver and a new transmitter. New developments in magnetic materials and the replacement of the single magnetic diaphragm by a balanced magnetic armature and separate aluminium diaphragm enabled the new receiver (Receiver Inset No. 4T) to have a considerably greater sensitivity and improved frequency response compared with its predecessor - Receiver Inset No. 2P. Unfortunately, no corresponding basic improvements which could lead to higher sensitivity had taken place in the carbon transmitter. Nevertheless, the low cost, robustness, and high sensitivity of the carbon transmitter had lead to its almost universal use in telephony, and development was mainly directed towards improving its quality of reproduction and its stability.

It was possible, therefore, when the new receiver became available to incorporate it in a new circuit using the Transmitter Inset No. 13, with the knowledge that a new transmitter could be used later without any modification of the circuit. With the introduction of the Telephone No. 706, it was also possible to incorporate a new handset (Handset No. 3) designed to work in conjunction with both the existing transmitter and the new transmitter as soon as it became available. This new transmitter, known as the Transmitter Inset No. 16 and illustrated in Fig. 1, is now being fitted on all new telephone instruments.



Although many of the performance characteristics required of the new transmitter can be specified in absolute terms, it is simpler to consider the design objectives in relation to the known achievements and limitations of the Transmitter Inset No. 13. The latter, which has been in almost universal use during the post war period both in the United Kingdom and many Commonwealth countries, has on the whole an excellent record of service, and the basic design has not been changed since its introduction about 30 years ago. The main performance characteristics of this transmitter and the improvements required to be achieved by redesign are as follows.

Increased sensitivity is generally desirable to give improved performance on long lines, but any marked improvement would require adjustment to the transmission or regulator circuit to avoid excessive output on short lines. For the reasons stated in the introduction, and the fact that, on the most commonly used cables, the 706-type telephone set using the No. 13 transmitter has a transmission limit at or above its signalling limit, only a marginal improvement in sensitivity was required.

Frequency Response
To obtain an overall talker-listener response as similar as possible to that obtained under direct conversation, it was desirable to achieve a slightly rising frequency response over the range 300-3,400 c/s. The frequency response of the No. 13 transmitter is resonant at about 1,400 c/s.

Poor articulation of certain sounds by the No. 13 transmitter may be attributed to its relatively high amplitude/ amplitude distortion. A considerable improvement was required.

Burning or Noise
All carbon transmitters are liable eventually to fail because of carbon or other deterioration causing noise, but the No. 13 transmitter has been reasonably satisfactory. Improvement, though desirable, was not essential.

Signalling Resistance
To meet the requirements of exchange signalling in Post Office subscribers’ circuits, it is essential that the signalling resistance of any transmitter should not exceed 300 ohms under the most adverse circuit conditions and throughout the life of the transmitter.

Stability of Performance
Short-term variations of sensitivity, resistance, and noise occur due to the manner in which the handset is held, to the ambient atmospheric conditions and to the transient conditions as the transmitter “warms up.” Additional long-term changes in performance, almost always adverse, occur due to the slow deterioration of the granular carbon or the electrode surfaces. The failure rate of No. 13 transmitters has not been excessive, but temporary losses of sensitivity after prolonged periods of rest have been observed in service. Because of the almost infinite variation of conditions which occur in use, laboratory testing alone cannot guarantee to detect all possible variations in performance, and extensive field trials are an essential part of transmitter assessment.


The design of a carbon transmitter may conveniently be considered as divided into two parts: first, the acousto-mechanical transducer that converts sound energy into mechanical displacement of the front electrode; second, the carbon chamber where this displacement compresses the carbon granules to produce resistance modulation.

The design of the acousto-mechanical transducer will control the frequency response and, to some extent, the sensitivity and amplitude/amplitude distortion of the complete transmitter. In its simplest form the diaphragm, electrode and carbon masses form a single series-resonant system with the sum of the stiffnesses due to the diaphragm flange, the air volume behind the diaphragm and the granular carbon. Unfortunately, both the effective mass and stiffness of granular carbon varies with the amplitude of displacement, so the design must ensure that their contribution to the total is sufficiently small to prevent high amplitude/amplitude distortion. The resonant response may be at least partially equalized by superimposing an anti-resonant system consisting of a second air volume behind the diaphragm, connected to the first through an acoustic hole or mass suitably damped. The resulting response is much smoother, but overall sensitivity is lowered and, as will be apparent later, some compromise is necessary. The holes in the handset mouthpiece and in the front cover of the transmitter, together with the air interspacings, form a low-pass filter. It is possible to adjust these to maintain output at higher frequencies, but this results in a fairly sharp drop in sensitivity above the cut-off frequency.

The carbon chamber design is influenced by so many complex interactions and often conflicting requirements that experimental work, past experience and manufacturing methods rather than calculation will largely determine the final configuration of the electrodes and boundary walls.

The general design will determine the sensitivity, the resistance, the distortion, the noise and the stability of the complete transmitter. Since electrode displacements imparted by the sound are exceedingly small, thermal expansions due to unequal or transient heating or ambient temperature change can be of the same order, so that materials must be chosen to eliminate differential expansions which could cause unwanted electrode movements.

Long-term stability is largely determined by the quality of the carbon granules, which normally tend to increase in resistance, become noisier, and decrease in efficiency during service. Considerable research has been carried out in this country and elsewhere to produce a more stable carbon. By suitable oxidation and mechanical pre-aging treatment it is possible to produce granules of greatly improved stability, and such granules have been employed in many telephone sets where a stable transmitter resistance is essential to the functioning of the automatic transmission regulator. Unfortunately, these pre-aged carbons have initial efficiencies somewhat lower than normal granules, and it has been considered that, for use in the 706-type of transmission circuit with a regulator action sensibly independent of transmitter resistance, the efficiency loss would outweigh the advantages. However, both research and field trials of various carbons are currently in progress, and, if satisfactory, new carbons will be brought into service later. It must be emphasized, however, that the risks inherent in introducing any new carbon without the most rigorous field trials are far too great, so progress is necessarily slow.

Lastly, as in all mass-produced articles, a very important consideration at all stages of the design is cost. In the No. 16 transmitter technical performance must be of prime importance, and because it is not always possible to predict stability or end-of-life behaviour with great certainty from laboratory tests, extreme caution must be exercised in the introduction of changes. Nevertheless, with the closest co-operation between the Post Office and the manufacturers, it has been possible to allow sufficient flexibility in the construction to give cost-saving ideas the chance of being tried out under conditions of full-scale production.



A cross-sectional diagram of the transmitter is shown in Fig. 2.

The front electrode is formed from thin sheet material, which forms into the required hemispherical shape with a fixing flange and allows the mass to be kept to a minimum. The electrode material is carbonised nickel manufactured from pure nickel strip by a series of processes which includes oxidation and subsequent replacement of the oxide by a pyrolytic deposition of carbon. The carbon deposit makes an excellent contact surface even after subsequent forming, and has the double advantage over plated surfaces of being considerably cheaper and involving no risk of plating salts contaminating the contact surface. It is, however, necessary to exercise extreme care during fabrication to avoid damage or contamination.
The shell-type back electrode is also manufactured from carbonised nickel strip.


Carbon Chamber
The general construction of the carbon chamber is apparent from Fig. 3, which shows that, by extending the wall of the back electrode outwards, it is possible to complete the chamber boundaries by three flat artificial silk washers through which the front electrode protrudes. All raw edges of the carbonised nickel are kept outside the carbon chamber.

To minimize variation in resistance when the transmitter is used in a handset, it is necessary to maintain a carbon filling of the order of 94 per cent of the total chamber volume. Since normal manufacturing tolerances lead to relatively large variations in the actual volumes of individual chambers, it is not easy to fill with a fixed volume charge, and special techniques are necessary to obtain a 94 per cent fill for each transmitter.

The carbon chamber is closed by a small nylon plug which is a push fit in the central filling hole of the back electrode.
The strictest precautions against contamination of the carbon chamber and granules are essential, and all materials, particularly plastics, within the transmitter have been carefully selected to prevent any deterioration due to the slow exudation of harmful vapours, etc.

Moisture from the breath has a highly corrosive action and must be prevented from reaching the carbon chamber, the diaphragm and the diaphragm-to-frame contact area. In the No. 13 transmitter an enamel finish is given to the diaphragm outer surface and a ring of rubber-based sealant is applied on both sides of the peripheral clamping ring. This has proved reasonably satisfactory, but there are manufacturing difficulties in ensuring a clean contact between the frame and the underside of the diaphragm, and best results have been obtained using expensive hand operations. Therefore, in the new transmitter protection has been obtained using a special formed-polypropylene membrane. A central conical depression closely couples the membrane to the diaphragm and eliminates any risk that the membrane might stretch in service and become stuck to the front plate; a turned-down lip round the periphery seals tightly on to the underside of the frame when the front cover is turned inwards, clamping the diaphragm at the same time. Some difficulty has been experienced in maintaining the rather close manufacturing tolerances necessary for this form of seal, and alternative constructions are being investigated; in fact, a modified design is already in use in a variant of the design - the Mark III transmitter, which is described in a later paragraph.

Other Constructional Features
The diaphragm is pressed from aluminium-magnesium alloy and carries the front electrode securely fixed at the apex of the cone. This assembly rests on a diecast aluminium frame, which carries the carbon chamber staked into its central well, and a washer of acoustic resistance material stuck over the three holes which form the mass of the acoustic equalizer.

The back cover carries the 6BA terminals and the nickel silver spring contacts which bear with considerable force on the frame and on the insulated back-electrode. Recesses in the back cover allow for quick and direct connexion of the handset-cord tag-ends. The enclosed volume of the back cover also forms the acoustic equalizing volume. A reasonable seal to the frame is required, but it is also necessary to provide a high resistance breathing hole to equalize the slow pressure changes caused by internal heating or ambient atmospheric conditions. A small central hole backed up by a felt plug is therefore provided; the resilience of the plug is also used as a safety device to prevent any possibility of the carbon-chamber nylon plug working loose in service.



Frequency Response and Amplitude/Amplitude Distortion - Fig. 4 (a) shows a typical frequency response measured under matched load conditions with constant free field applied sound pressures of 30, 10 and 3 dynes/cm2. The improved frequency response and lower amplitude! amplitude distortion compared with that of the Transmitter Inset No. 13C shown in Fig. 4(b) are immediately apparent. Nevertheless, the response falls off rather than rises at higher frequencies as required by the design objective, but there are two reasons for this. First, a compromise had to be reached between loudness and quality, and to maintain the former some peakiness remains in the region of 1,300 c/s. Secondly, a dip in the frequency response between 2 and 2-5 kc/s is advantageous in avoiding any possible speech interference with the C.C.I.T.T. signalling frequency of 2,280 c/s.

Transmission Performance
As might be expected from the design objectives the loudness efficacy and talking resistance of the No. 16 transmitter are substantially equal to those of its predecessor. On conversational opinion-rating tests the advantage of the new transmitter is of the order of 5 db. Articulation tests also show substantial improvement in certain final consonants for which the No. 13 transmitter has a relatively poor performance.


Field Trials
Concurrently with laboratory testing, four separate forms of service trial had been undertaken satisfactorily before full-scale production was authorized.

First, limited numbers of No. 16 transmitters have been placed in selected public call-offices on main-line railway stations, where usage is virtually continuous throughout the day and evening, and the atmosphere is liable to be adverse. By making periodical laboratory measurements of frequency response, resistance and noise, it is possible to ascertain within 12 months the deterioration trends. The average life of 2 years obtained under these conditions is considered satisfactory.

Secondly, similar controlled tests were carried out with transmitters placed in selected busy telephones in a commercial office. Under these conditions the measurable deterioration after one year’s service was nil, and there were no transmission complaints such as would be expected if the performance suffered from any short term instabilities.

Thirdly, several thousand transmitters were fitted at random into new subscriber trunk dialling public-call offices. This test lacks, of course, the precision of controlled tests, but allows a much larger scale of testing with wider and more random distribution of operating conditions.

Lastly, No. 16 transmitters have been used to replace No. 13 transmitters where adverse transmission conditions were known to exist because line lengths exceeded normal, or where specific transmission complaints have been made.

Manufacturing Testing
For manufacturing testing it is necessary to limit the number of tests which can be made on each transmitter and to rely on general quality control and batch sampling for checking other parameters. In general, tests for sensitivity, frequency response, resistance and sealing will be carried out on all transmitters. The first three parameters may be checked simultaneously by the use of a white-noise band tester, which applies sequentially to the transmitter, through an artificial mouth, a broad band of noise covering the frequency range 300-3,400 c/s, followed by three narrow bands of noise positioned in the lower, middle and upper portions of the wideband spectrum. Sensitivity can be checked by measuring the mean output whilst the transmitter is energized by the broad band, and control of frequency response is maintained by allowing only very narrow spreads in the outputs of the three narrow bands relative to the particular broad-band output of the transmitter under test. Dynamic resistance may be checked during this test.


The transmitter is held in contact with the mouthpiece by a spring ring similar to the one used with the Receiver Inset No. 4T. The spring ring, supported by moulded lugs in the handset, presses against the back surface of the transmitter. The rotation of the transmitter is prevented by lugs which are positioned by ribs on the inside of the mouthpiece cavity.
The transmitter is about 0.9 oz less in weight than the Transmitter Inset No. 13. The adoption of ABS material for the handset mouldings will further reduce the weight of the Handset No. 3 to about 7.75 oz, excluding the cord. This worthwhile reduction can be passed on to the subscriber in all but the very few instances when the telephone has been fitted with add-on gravity-switch spring-sets with the maximum number of springs. For these telephones a small weight can be added to the handset to ensure the satisfactory operation of the gravity-switch mechanism.


Two modifications to the original design (Mark 1) have been developed to permit manufacturers to make full use of their own production techniques. Both designs are physically interchangeable with the Mark I and have similar electrical performances.

The Mark II transmitter (Fig. 5) has a modified back cover which extends over the frame and is clamped by the rim of the front cover, thus making it unnecessary to have the three back-cover fixing screws.

The Mark III transmitter, which externally is identical to the Mark I design (Fig. 1), has a modified diaphragm assembly. The moving electrode is sealed to the diaphragm, which is protected by enamel, So that it is no longer necessary to have a membrane in front of the diaphragm. The edge of the diaphragm is sealed with a pliant ring and sealing compound.



The Transmitter Inset No. 16 is now superseding the Transmitter Inset No. 13C for all 700-type telephones. Its slightly improved sensitivity, better frequency characteristics, lower amplitude/amplitude distortion, and freedom from angular positional effects will be of particular advantage under adverse transmission conditions; in all circumstances it will help to bring better quality to telephone speech. With its introduction into the 706-type telephones, the development of this telephone may now be said to be complete and it can the more readily take its place among the few first-grade telephone sets in general use throughout the world.

Only the test of time can determine beyond all doubt just how satisfactory in service the new design will prove to be, but in these days of rapidly increasing knowledge in the field of semiconductors, it could well be that the Transmitter Inset No. 16 could remain in production with only relatively minor design variations until the era of the carbon microphone for general telephonic use has ended.

The new transmitter has been developed by Standard Telephones and Cables Ltd., for the Post Office under the British Telephone Technical Development Committee procedure.

‘SPENCER, H. J. C., and WILSON, F. A. The New 700-Type Telephone. P.O.E.E.J., Vol. 49, p. 69, July 1956.
‘ROBERTON, J. S. P. The Rocking-Armature Receiver.
P.O.E.E.J., Vol. 49, p. 40, Apr. 1956.
SPENCER, H. J. C., and WILSON, F. A. The New 700-Type
Table Telephone - Telephone No. 706. P.O.E.E.J., Vol. 52, p. 1, Apr. 1959.


BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: January 15, 2011