430 FEAGMENTS OF SCIENCE. 



the handle of the machine is rapidly turned the wire 

 glows, first with a red heat, then with a white heat, 

 and finally with the heat of fusion. The moment the 

 wire melts, the circuit round the armature is broken, 

 an instant relief from the labour thrown upon the 

 arm being the consequence. Clearly realise the equiv- 

 alent of the heat here developed. During the period 

 of turning the machine a certain amount of combusti- 

 ble substance was oxidised or burnt in the muscles 

 of my arm. Had it done no external work, the mat- 

 ter consumed would have produced a definite amount 

 of heat. Now, the muscular heat actually developed 

 during the rotation of the machine fell short of this 

 definite amount, the missing heat being reproduced 

 to the last fraction in the glowing platinum wire and 

 the other parts of the machine. Here, then, the elec- 

 tric current intervenes between my muscles and the 

 generated heat, exactly as it did a moment ago between 

 the voltaic battery and its generated heat. The electric 

 current is to all intents and purposes a vehicle which 

 transports the heat both of muscle and battery to any 

 distance from the hearth where the fuel is consumed. 

 Not only is the current a messenger, but it is also an 

 intensifier of magical power. The temperature of my 

 arm is, in round numbers, 100 Farm, and it is by 

 the intensification of this heat that one of the most 

 refractory of metals, which requires a heat of 3,600 

 Fahr. to fuse it, has been reduced to the molten con- 

 dition. 



Zinc, as I have said, is a fuel far too expensive to 

 permit of the electric light produced by its combustion 

 being used for the common purposes of life, and you 

 will readily perceive that the human muscles, or even 

 the muscles of a horse, would be more expensive still. 

 Here, however, we can employ the force of burning coal 



