284 MR JAMES BLYTH ON THE TRANSMISSION OF SOUND. 
telephone by means of a microphone transmitter. It is well known that these 
can be heard with a very weak battery in the circuit, and even with no battery 
at all, provided the points of the microphone carbon be alittle moist. I find that 
sounds can be heard in the telephone without a battery, and with the carbons 
apparently quite dry, if we rub the carbons hard together. This rubbing is 
distinctly heard, and it seems that it must arise in part at least from thermo- 
electric currents produced by the friction. That such are produced is readily 
shown by attaching two wires to the terminals of a THomson’s reflecting gal- 
vanometer, and to the ends of these wires any two conducting substances. 
When these substances are rubbed against each other the movements of the 
spot of light clearly indicate the production of currents. I have roughly tested 
these currents, and find that they are stronger in proportion as the metals 
rubbed are wider apart on the theremo-electric scale ; but I have found no two 
substances, even of the same kind, which do not give them slightly. It is just 
possible, however, that such currents may not be wholly thermo-electric, but 
that some may be due, as I mentioned in a recent paper to the Society, to the 
currents suggested by Sir WiLL1Am THomsoN as the cause of friction. 
