VIII.—On the Transmission of Sound by Loose Electrical Contact. 
By JAmes Brytu, M.A. 
(Read 27th July 1879). 
In a paper published in the Transactions of this Society for Session 1877-78, 
I described an experiment which showed, that if a moderately strong current, 
such as that from four or five BUNSEN cells, be led through two jam-pots filled 
with fragments of carbon, and if any sound be uttered strongly in the one jam- 
pot it will be reproduced distinctly, although faintly, in the other. In this 
| experiment it has been found that the fragments of carbon may be replaced by 
| any kind of loose contact, such as microphones, or a handful of screw-nails put 
| into each jam-pot, or vibrating springs beating against metallic stops, or nails 
| laid across each other in log-hut fashion, and that in each case an effect similar in 
kind, although it may be differing greatly in degree, is produced. Hence it 
| may be almost laid down as a general experimental result, that if an electric 
circuit conveying a tolerably strong current contain two places of loose contact, 
_ A and B, and if any sound be produced loud enough at A a similar sound will 
| be heard proceeding from B. 
| To all appearance this phenomenon can only arise from the altered resist- 
| ance produced at A by the sound waves, and it becomes a problem to explain 
| how this altered resistance at A so affects the materials in contact at B as to 
' make them give forth waves which convey a similar sound to the ear. No 
_ satisfactory solution of this problem has as yet been given, and it was in hopes 
of getting some information on the subject that I made the following experiments. 
| Experiment 1.—Four strong Bunsen cells were included in the circuit, and 
| the loose contacts A and B placed in different rooms, so that the sound uttered 
at A could not be directly heard at B. (Throughout we shall understand by 
A the sending, and B the receiving station.) In order to make the alteration of 
| resistance at A as great as possible, an actual make-and-break was there inserted. 
| A toothed wheel driven round against a spring or any one of the ordinary loud- 
_ sounding automatic kinds would do; but what served my purpose best in this 
experiment was made in the following way :—One of the terminal wires of the 
circuit was firmly attached to a tin can and the other to a common round file. 
A hole was then pierced through the bottom of the can at its centre, and the 
file driven backwards and forwards in the hole as if for the purpose of making 
it larger. At the receiving end B a precisely similar can and file were used, 
and the file allowed to rest lightly in the hole. Every to-and-fro rasp of the file 
VOL, XXIX. PART I. 4 C 
