PREVENTING THE TELEPHONE FROM SLIPPING
From the introduction of the first table telephone the form and material of the feet presented a problem. With the early instruments, the difficulty was to prevent the marking of the surfaces on which the telephone stood, but when a dial was added to the instrument, it became necessary to ensure that the instrument did not slip during dialling. Similarly, the recent innovation of the extensible coiled H.M.T. cord has made it necessary to prevent the pull of the cord from dragging the instrument off the table and dropping it on the floor. The most unfavourable combination from this point of view is a lightweight instrument with a stiff extensible cord.
|A - The Present Standard Domed Foot B - The New Foot|
Although the elementary theory indicates that the sideways pull which the telephone will withstand depends only on the materials of the feet, the table surface, and the weight of the instrument, there are in practice a variety of complicating factors, notably the shape of the feet and the condition of the surfaces as regards dust, furniture polish, etc. Furthermore, both the shape and the material of the feet affect the degree of staining and marking occasioned by an instrument.
To improve the feet of the Etelphone, the Company’s Process Laboratories have made an extensive study of the feet standardized by the B.P.O. for the Telephone 706 and have compared them with other shapes and materials. From this has come a new foot and a good measure of confidence that it is a marked improvement on the old.
Always it is necessary to accept a compromise between the requirements of not slipping and not damaging furniture surfaces. A foot that is right for all possible conditions is, therefore, unlikely; nevertheless, the new foot should be generally satisfactory. However, there are some relatively soft surfaces which any generally acceptable foot is almost certain to damage, and the only satisfactory answers to these cases are to protect the surfaces by other means or site the telephone elsewhere.
|A - The existing domed foot showing the large area of dust and wax accumulation|
|B - The new foot in white Butakon, illustrating the clean edge|
| Figure 2
Examples of Dirt Adhesion on Existing and New Feet
THE OLD AND THE NEW FEET
When the current standard domed feet (see Figure 1A) are new and clean, adhesion is generally satisfactory, and the horizontal force necessary to drag a telephone over a typical table surface may be as high as 50-60 ozs. which is more than adequate to resist the 12 ozs. pull of an average coiled cord of the original stiff pattern. In use, however, dust, dirt and wax accumulation on the foot (see Figure 2) reduced adhesion to a remarkable degree. After a month of normal use it may be as little as 6 ozs. Figure 3 shows as a pillargraph the results of tests on 78 typical instruments in ordinary use at Beeston factory.
To reduce the chances of the cord pull dragging the telephone over the table, two steps have been proposed.
1. To replace the stiff coiled cord by a new pattern which requires a pull of only 6 ozs. to extend it to 4 feet.
2. A form of foot (see Figure 1B) with a ‘cup’ profile which gives a ‘feather-edge’ on which the instrument rests. It was hoped that this would give a self-cleaning effect and as will be seen from Figures 2A and B, this hope seems to
have been realized. While dust and dirt accumulate on the faces of the foot the featheredge seems to remain clean.
Three grades of rubber were tried with the new shape, viz:-
(a) Chloroprene rubber complying with B.P.O. Specification M.105A (ref. B.369).
(b) A softer grade of chioroprene (ref. B. 139).
(c) Butakon, a synthetic elastomer.
Results of the test on 78 Instruments, with old feet, in ordinary use in the |Beeston Factory
Typical Degrees of Staining
Method of testing pull
To study the problem and the effect of the proposed shape of foot, a series of tests were made in the Laboratory and on instruments in service at Beeston.
The general arrangement for the Laboratory tests is shown in Figure 5 and consists of an elementary means of measuring the smallest force to move the telephone. The tests were conducted with three ‘table’ materials with the two angles of pull and under clean and dusty conditions.
In every case the effect of dust and dirt was to reduce the adhesion but the reduction was most marked with the old feet and least with the new shape in B.139 and B.369.
For the telephones in service the pulley and weights were replaced by a spring balance and the initial results on 78 telephones with the old pattern feet are as already indicated in Figure 3. The following table summarizes the results obtained with 16 instruments fitted with new pattern feet of B. 139.
AFTER NORMAL USE
|NEW||1 MONTH||2 MONTHS||3 MONTHS||4 MONTHS|
All pulls in ozs.
MARKING AND STAINING TESTS
To establish how the new foot compared with the old in its effect on surfaces, tests were conducted at 20°C. 75% R.H. and 40°C. dry in which old standard feet and the new shape of foot in the three grades of rubber were loaded with weights equal to a quarter of that of the telephone instrument and were stood on a variety of table surfaces for 35 days without movement.
The surfaces included were:-
Shellac (French polish)
Celluloses; two formulations
Enamels; two formulations
Catalysed Synthetic Resin
Linoleum; Waxed and Unwaxed
It is impossible to reproduce all the photographs showing the degrees of marking or indentation of the surface but Figure 4 indicates three degrees of staining. Using classifications of this sort it was possible to decide on the general placings of the materials and shapes of foot.
New feet of B. 139 were markedly inferior to those of B.369 as regards marking and staining, the latter being generally compatible to the old domed foot in this respect.
Out of this work comes a new shape of foot in B.369 rubber which it is confidently expected will be markedly superior to the existing standard foot as regards frictional properties and not noticeably worse in its liability to stain and mark furniture surfaces.
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