
( 557-) 
XVIII.—An Account of some Experiments on the Telephone and Microphone. 
By James Brytu, M.A. 
(Read 3d June 1878.) 
The first experiment which I have to describe relates to an attempt to 
employ the telephone as a means of measuring the hearing power of the ear in 
different individuals, or in the same individual at different times. For this 
purpose a constant source of sound, driven by clockwork, is made to act upon 
the disc of a telephone in a distant room. In the circuit of this telephone are 
included two others, so placed on a stand, that they can be brought close to 
both ears of the person to be experimented on. Soft india-rubber rings are 
attached to these telephones so as to enclose both ears, and serve the double 
purpose of avoiding any disagreeable pressure on them, and preventing, as far 
as possible, all extraneous sounds from entering. In the circuit of the 
telephones there is also included a column of slightly acidulated water whose 
length can be varied at pleasure, so as to introduce a greater or less resistance 
to the electric current. This is managed in the following way :—A pretty wide 
glass tube, about 24 centimetres long, closed at the lower end, and graduated 
to millimetres, is mounted vertically on a stand. Projecting through the lower 
end of the tube is a platinum wire melted into the glass and having its point 
inside opposite the zero of the scale. By means of a rack motion another 
platinum wire is made to move up and down the axis of the tube through its 
entire length. This tube is filied with water, containing a few drops of 
sulphuric acid, and is placed in the circuit of the telephone by means of the 
platinum wires. When the instrument is used the platinum wires are first 
placed in contact, so that the clockwork sound is clearly heard in the telephones. 
The platinum wires are then gradually separated, introducing more and more 
of a liquid resistance into the circuit, the sound meanwhile getting fainter and 
fainter, until a point is reached when no sound at all is heard. At that point 
the distance between the points of the platinum wires as read on the milli- 
metre scale, gives the measure of the hearing power of the ear for the 
particular individual examined. Tested in this way the hearing power of no 
two persons will be found to be identical, while even in the same person it 
varies remarkably at different times. It should be noted, however, that the 
reading will be different for the same individual on a first trial from what. it 
will be after he has got a sort of training in the hearing of telephonic sounds. 
VOL, XXVIII. PART II. rg 
