Transmitters are generally based on the Hughes design but can be categorised into three general classes - the Button Carbon (prototype - Blake), the Pencil Carbon (prototype - Crossley) and the Granulated Carbon (prototype - Hunnings).
Telephone transmitter inventers in this document:-
The transmitter is fitted to the stand by means of a recessed sleeve, G, fitting into the collar, H, of a separate stand, connection being made to the two terminals, I and J, by the springs, K and L, the later being insulated and bearing on the end of the adjusting screw, D.
The base, M, of the stand is of thick metal and the weight of the upper part, N, and the transmitter is borne on three springs, S, S, S, the centres of which are fastened to N and the near ends clamped by screws to M, whilst the far ends are free to slide in recesses formed in M. The springs absorb vibrations from the stage.
This transmitter was used on stage for the Electrophone system in the UK.
Francis Blake from the USA used the results of the Hughes experiments to produce an efficient transmitter in 1877. His patent was bought by the Bell company and the transmitter was widely used in the USA. It was also used in the UK, but had a dubious legal status as it used carbon granules just like the Edison transmitter. This was resolved when the Bell and Edison companies merged in the UK in 1880.
The Blake transmitter could be mounted in a wooden or metal case, which also included the induction coil. In most cases the casing was wooden made to a standard pattern which is easily recognised and was labelled "Blake Transmitter" on the front.
This transmitter uses a small pellet of platinum, which is attached to a light spring (f) and at the other end anchored to an iron yoke. Another spring (g) is also anchored to the iron yoke and fixed to the end of the spring is a brass socket (p) with a carbon block attached (k). The platinum pellet is so placed that it in contact with the iron diaphragm (e) and the carbon block (k). The pressure on the platinum pellet is regulated an adjusting screw (n) on the yoke. These two springs form the electrodes for the primary current and the inertia of the brass socket causes variations of pressure between the platinum pellet and the carbon when the iron diaphragm (e) is vibrated by voice waves.
Louis Crossley, of Halifax in the UK, designed a transmitter based on the Hughes carbon-pencil transmitter (1879). He used four loose pencils of carbon and mounted these in a diamond formation between hollows in four carbon blocks that were secured to the underside of a thin pine wood diaphragm. The transmitter can be seen in the picture below.
Blakey and Emmott of Halifax used this transmitter in their telephones and were the first manufacturer to supply the Post Office in 1881.
W. Deckert of Austria modified this granular transmitter which was manufactured by GEC and used by the British Post Office as a replacement for the Gower transmitters they were using at the time. It is also referred to as the Hunnings-Cone Transmitter.
Some private telephone companies also used this transmitter to replace the Blake transmitters they had in use - often fitting them in place of the original Blake transmitter. Because of this it cannot be assumed that a Blake transmitter box actually contains a Blake transmitter!
The major difference from other carbon granule transmitters is that the front face of the fixed electrode (c) is formed into a number of square based pyramids. The pyramids of one row are opposite the middle of the pyramids above. Only the centre portion of the carbon electrode is used, with the unused part being covered with a thick pad of cotton wool (L). The tips of the pyramids in the centre portion of the carbon electrode are flattened and little tufts of floss silk are attached. These prevent the carbon electrode short circuiting against the outer carbon diaphragm (d) and help to prevent vibrations and packing of carbon granules. At the front of the mouthpiece is a wire gauss membrane (g) to protect the delicate carbon diaphragm.
This transmitter is not free from the issues of packing and is so designed that the transmitter may be rotated half a turn every so often as a preventative measure.
In 1880 L. M. Ericsson redesigned the Blake transmitter. This new design was smaller than the original and adjustments to it were made by a micrometer type screw, or helix. Thus the name Helical transmitter.
The transmitter is constructed as thus - the horn shaped sound tube (s) has at it's base a membrane (w) which has a carbon disc (v) fastened to it's centre. This carbon disc is has a platinum stud pressing on it, carried by a rod which is also pressed by a spring. A second similar carbon disc and platinum stud is made at the lower end. The micrometer style ended screw (m) regulates the pressure of the contacts.
Carbon Transmitter (N.T. 10)
This is a highly successful transmitter but there are a number of variants. The picture below gives a sectional view of one of the better variants.
The transmitter comprises of an aluminium casing (a and b) made in two parts. The back part of the case (b) has a recess, in which fits a 7/8 inch round carbon block (c), 1/4 inch thick. The front face of this block has seven holes drilled in it - one in the centre, in which fits the head of the long, clamping and connecting screws, and six other holes, as to the right of the picture (which gives a separate view of the carbon block).
Tufts of cotton wool are fitted in each of the seven holes. These tufts, when in position, press on the carbon diaphragm (o), which is of thin carbon, 2.25 inches in diameter and 40 mils thick. A sort of sleeve of soft felt (f) fits over the edge of the carbon block and is also pressed against the diaphragm (d) by the heads of six bronze springs which are all joined together at the centre and clamped with the carbon block. Six recesses are made in the latter to allow the springs to work freely. The carbon block and screw (s) are otherwise insulated from the case (b), by a mica washer (m) and bush (h), and by an ebonite washer (w), at the back.
Rings of blotting-paper (p) are used to clamp the diaphragm and they also serve to clamp in front a membrane of oiled silk (k), which prevents the moisture of the breath, etc., reaching the diaphragm.
About five grains of carbon granules (g), are put in the recess left at the back of the diaphragm. The normal resistance of this transmitter is about 100 ohms and in working it varies between about 50 and 170 ohms.
Another form of Ericsson is similar to this as regards the carbon block, etc. (except that the six outer holes in the block are missing); but the diaphragm is of ferrotype iron (about 12 mils thick), on the inner face of which is fitted a gilded thin metal dish, the surface of which has about thirty eight indentations pressed into it in concentric rings. These indentations are projections on the other side. They project into the carbon granules, making a good contact and preventing packing.
The Gower transmitter is a variation on the Hughes carbon pencil microphone, but consists eight carbon pencil rods between nine carbon blocks. These are arranged in a star formation and eight blocks are connected to two copper strips. This forms two groups of pencils, in series, with each group consisting of four parallel pencils. The transmitter is shown to the left of the picture below.
Born in London, UK, David Hughes investigated the operation of the carbon granule transmitter. He made the accounts of his investigations freely available and did not intend to profit from them. Hughes discovered transmitter worked by variations in the resistance of the carbon granules caused by sound and not compression of the carbon, as others thought.
He did not produce a commercial transmitter but his experiments on carbon granule and pencil transmitters that paved the way for others. Blake was one of many who made a successful transmitter based on Hughes experiments.
The first of the granular type of carbon transmitter was invented by English clergyman, Rev H. Hunnings, in 1878 (British Patent Specification 3647). The original design consisted of powered coke between a platinum diaphragm and a brass plate. Later designs used hard carbon granules which were located between two platinum electrodes. Although an improvement on other carbon granule transmitters it suffered badly from granule packing. W. Deckert took this design and modified it to greatly reduced packing and give superior sound reproduction.
Resting in a central cavity of an ebonite seating is a carbon block, C, with a face moulded into a number of pyramidal projections, P P. The space between C and a carbon diaphragm, D, is packed with carbon granules. C has direct contact with line terminal T, which screws into it; D with T1 through the brass casing, screw S, and a small plate at the back of the transmitter. Voice vibrations compress the carbon granules and allow current to pass more freely from D to C.
This form of microphone is very delicate, and unequalled for long-distance transmission.
White Transmitter (Solid Back)
Known as the solid Back transmitter this was invented by an American called A. G. White. This was the standard transmitter for Bell Companies operating in the United States and also in other parts of the world. The BPO called this the Transmitter No. 1 (Mark 4001).
This transmitter was in general use on C.B. exchanges in the early 1900's and was only really superseded by the high efficiency and low cost of the new style carbon granule transmitter in the last 1920's.
The working parts are all mounted on the front casting (1). This is supported in a cup (2), in turn supported on the lug (3), which is pivoted on the transmitter arm or other support. The front and rear electrodes of this instrument are formed of thin carbon disks shown in solid black. The rear electrode, the larger one of these disks, is securely attached by solder to the face of a brass disk having a rearwards projecting screw-threaded shank, which serves to hold it and the rear electrode in place in the bottom of a heavy brass cup (4). The front electrode is mounted on the rear face of a stud. Clamped against the head of this stud, by a screw-threaded clamping ring (7), is a mica washer, or disk (6). The centre portion of this mica washer is therefore rigid with respect to the front electrode and partakes of its movements. The outer edge of this mica washer is similarly clamped against the front edge of the cup (4), a screw-threaded ring (9) serving to hold the edge of the mica rigidly against the front of the cup. The outer edge of this washer is, therefore, rigid with respect to the rear electrode, which is fixed. Whatever relative movement there is between the two electrodes must, therefore, be permitted by the flexing of the mica washer. This mica washer not only serves to maintain the electrodes in their normal relative positions, but also serves to close the chamber which contains the electrodes, and, therefore, to prevent the granular carbon, with which the space between the electrodes is filled, from falling out.
The cup (4), containing the electrode chamber, is rigidly fastened with respect to the body of the transmitter by a rearwards projecting shank held in a bridge piece (8) which is secured at its ends to the front block. The needed rigidity of the rear electrode is thus obtained and this is probably the reason for calling the instrument the solid-back. The front electrode, on the other hand, is fastened to the centre of the diaphragm by means of a shank on the stud, which passes through a hole in the diaphragm and is clamped thereto by two small nuts. Against the rear face of the diaphragm of this transmitter there rest two damping springs. These are shown in the rear view picture. They are secured at one end to the rear flange of the front casting (1), and bear with their other or free ends against the rear face of the diaphragm. The damping springs are prevented from coming into actual contact with the diaphragm by small insulating pads. The purpose of the damping springs is to reduce the sensitiveness of the diaphragm to extraneous sounds. As a result, the White transmitter does not pick up all of the sounds in its vicinity as readily as do the more sensitive transmitters, and thus the transmission is not interfered with by extraneous noises. On the other hand, the provision of these heavy damping springs makes it necessary that this transmitter shall be spoken into directly by the user.
The sides of the brass cell (4), are lined with paper, and the space left is filled about three-fourths full with carbon granules. If, however, the transmitter is to be used for common battery (C.B.) working, only about one-half the amount of granules (88 grains) is used, in order to increase the resistance.
The action of this transmitter is as follows:-
Sound waves are concentrated against the centre of the diaphragm by the mouth-piece, which is of the familiar form. These waves impinge against the diaphragm, causing it to vibrate, and this, in turn, produces similar vibrations in the front electrode. The vibrations of the front electrode are permitted by the elasticity of the mica washer (6). The rear electrode is, however, held stationary within the heavy chambered block 4 and which in turn is held immovable by its rigid mounting. As a result, the front electrode approaches and recedes from the rear electrode, thus compressing and decompressing the mass of granular carbon between them. As a result, the intimacy of contact between the electrode plates and the granules and also between the granules themselves is altered, and the resistance of the path from one electrode to the other through the mass of granules is varied.
The diaphragm is 2.5 inches in diameter and .022 inches in thickness with the carbon cell being 5/8 inch internal diameter.
The normal resistance of the ordinary instrument is about 30 ohms and the C.B. variant is about 55 ohms.
Last revised: June 02, 2021