OF ABTS AND SCIENCES. 89 



tion upon the current within the coils, — whereas with permanent 

 magnets there is only the first magnetic induction. 



Improvements in this direction can hardly be expected, further than 

 to utilize fully the currents which we already get ; and these, I am 

 sure, are not fully utilized. For it is not an uncommon experience 

 with my instruments, that the vibrations are strong enough to be very 

 perceptible by the hand upon the instrument ; and thus it appears to 

 be like a tuning-fork held in the hand : when struck, the sound is not 

 given out to the air, but is smothered as it were in the hand ; but, 

 unlike the fork, it will not be perceptibly louder when placed upon a 

 large resonant surface. It is not improbable that much of this spent 

 energy manifests itself as heat in the plate ; but I have not yet 

 investigated this. 



There is no third method of varying these primal conditions. These 

 two methods cover the ground of varying the current by means of a 

 change in the electro-motive force. 



With the other term, resistance, the case is very different. A 

 change in resistance may be effected by lengthening the conductor, 

 by changing its cross section, by interposing various substances having 

 different conductivities, by varying the density of a loose-grained con- 

 ductor, and in other ways, each one of the methods mentioned being 

 capable of application in very many different ways. Hence, tele- 

 phones constructed upon the plan of varying the resistance have been 

 invented in great variety, and by many different persons. Histori- 

 cally, Page, of Salem, stands first ; his automatic interruptor being 

 still employed for many purposes, and, when used as he used it in 

 connection with an electro-magnet as a receiver, it was a genuine 

 telephone. This was in 1837. Next, Reiss, of Germany, varied the 

 resistance by platinum terminals when contact pressure was variable ; 

 this was in 1861. It would seem that if Reiss had tried an ordinary 

 relay, or some such instrument for a receiver, in place of the one 

 which he did use, he would have had then a good speaking telephone. 

 What was lacking then, was an armature to his receiving magnet. 

 Gray, of Chicago, in February, 1876, used water for the same purpose. 

 In August, same year, I varied the cross section in mercury with the 

 iron cone. In January, 1877, Edison adopted carbon in the form of 

 lamp-black between a vibrating plate and a stiff backing. In Decem- 

 ber, 1877, I varied the contact pressure between the battery elements, 

 and still later Hughes used free carbon saturated with mercury, 

 and called his instrument a Microphone. The so-called microphone 

 does not differ in any essential thing from the other telephones 

 which have been in use for a good while. The name microphone 



