OP ARTS AND SCIENCES. 91 



Thus it is seen that the two different methods of producing speak- 

 ing telephones are as unlike each other as a battery is unlike a mag- 

 neto-electric machine, which, though they have the same function, — - 

 namely, to produce electricity, — there is nothing else that is common 

 They may be compared for efficiency, not for involved principles. 



Suggested Uses. 



I see no reason why the variability of conductivity due to pressure, 

 as in the case of carbon is so marked, might not be used in many places 

 where now are such instruments as thermometers and barometers. 

 Thus the expansion and contraction of an ordinary copper rod, or, 

 better still perhaps, a thin copper tube, might be made to act upon a 

 piece of carbon in circuit with a constant battery, and the galva- 

 nometer needle would constantly indicate the temperature if properly 

 calibrated. 



In like manner, the varying pressure of the atmosphere could be 

 made to be much more apparent than now. Perhaps a wind-gauge 

 would also be possible. Especially would it be possible to measure 

 the depth of water by its pressure upon a carbon disk when sunk into 

 the water. Here it would be necessary to have a double-line wire 

 through which the circuit could be maintained during the whole time 

 of the descent ; that is to say, the length of wire in the circuit would 

 need to be constant. If the temperature of the water be known, as 

 it generally is, the depth would be easily read off from the galva- 

 nometer. 



