18 On the Electrical Resistance of Carbon Contacts. [Feb. 1, 



fall in the resistance if the metal is clean, and by a small rise in the 

 resistance if the metal is not clean. 



Increased pressure produces a greater fall in the resistance with 

 small pressures than with great pressures, and with weak currents 

 than with strong currents. 



The resistance after having been reduced by increased pressure does 

 not return to its original value when the added pressure is removed. 



17. Reasons for the Superiority of Carhon over Metal in the Microphone. 



The above observations may, perhaps, throw some light on a matter 

 which has never hitherto been fully explained. Why does carbon 

 give far better results than any metal when used in the microphone ? 

 It seems to me that this question may be answered without much 

 difficulty. The mere fact that a current causes delicately adjusted 

 metal contacts to adhere to each other is sufficient to account for the 

 superior efficiency of carbon. A metal microphone might, indeed, be 

 used to transmit the pitch of a sound, provided that its vibrations 

 were sufficiently powerful to cause actual separation of the points of 

 contact. The fundamental tone might, in this way, be conveyed, but 

 it is clear that the minute superimposed vibrations, due to the upper 

 partials, upon which depends the distinctive character of a particular 

 sound, would be very imperfectly represented if not entirely lost. 



In addition to this phenomenon of adhesion, and probably connected 

 with it, are the facts that metallic contacts, unlike those of carbon, 

 do not even approximately recover their original resistance when once 

 it has been reduced by increased pressure or increased current, unless, 

 indeed, complete separation occurs : and even the initial effect of 

 pressure upon resistance is (except with very weak currents), in 

 general, much more marked with carbon than with metals. 



Lastly, it is to be noticed that in the case of carbon, pressure and 

 current act in consonance with each other : pressure diminishes the 

 resistance, and in so doing increases the strength of the current ; and 

 the current thus strengthened effects a further diminution in the 

 resistance. In the case of metals, on the other hand (or at least in 

 the case of clean bismuth) pressure and current tend to produce 

 opposite effects. The resistance is diminished by pressure and the 

 current consequently strengthened, but by reason of the increased 

 strength of current, the resistance is higher than it would have 

 been if the current had remained unchanged. The effect of this 

 antagonism is not very great, but it seems to give a material advantage 

 to carbon.* 



* In April, 1882, I communicated this observation to Mr. Preece, who referred 

 to it in a paper, read at the Southampton Meeting of the British Association, on 

 " Recent Progress in Telephony." 



