408 



Mr. W. H. Preece. 



[May 27, 



May 27, 1880. 



THE PRESIDENT in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. 4k On some Thermal Effects of Electric Currents." By 

 William Henry Preece, General Post Office. Communi- 

 cated by Professor Stokes, Sec. R.S. Received April 28, 

 1880. 



I have been engaged for some time past in experimenting on the 

 thermal effects of electric currents, but the final results of those experi- 

 ments are not sufficiently ripe at present to justify my bringing them 

 before the Royal Society. I have, however, obtained one result which 

 I believe to be sufficiently novel to justify a short preliminary note. 



The most striking facts elicited by these experiments are : 



1. The extreme rapidity with which thin wires acquire and lose 

 their increased temperature. 



2. The excessive sensibility to linear expansion which fine wires of 

 high resistance evince. 



Now as the rate of heating, and therefore of expansion and contrac- 

 tion, varies very nearly directly as the increment or decrement of the 

 currents, when these variations are very small, it occurred to me that 

 if a long wire of small diameter and high resistance were attached to 

 a sounding board, or to the centre of a disk (such as one of those used 

 for telephones and phonographs) and it formed part of a circuit con- 

 veying telephonic currents, sonorous vibrations ought to be repro- 

 duced. 



The sketch shows the arrangement of the apparatus used for the 

 experiment. 



A was a stout base of mahogany, on which a brass support C was 

 attached so that it could slide and be fixed at any distance from D. 



D was at first a disk of thin paper, and then of thin iron. 



P was the wire experimented upon whose loose ends were connected 

 to terminals on the wooden base, so as to be inserted in the circuit 

 containing a microphone transmitter M and a battery B of six bichro- 

 mate of potash cells in another room out of hearing. 



A platinum wire of 0*003 inch diameter and six inches long from j) 

 to j/ was first used, and the sonorous effects were most marked and 



