Distribution of Energy by the Electric Current, 355 



millim. diameter is attached at one end to an adjusting- 

 screw, B, and at the other to one end of a bell-crank lever, 

 L, by means of which the pressure is brought to bear upon a 

 pile of carbon disks, C, placed in a vertical glass tube. The 

 current enters the instrument at the adjusting-screw B, and, 

 passing through the wire and bell-crank lever, leaves below 

 the pile of carbon disks. Its effect is to cause a rise of tem- 

 perature in the steel wire, which, through its expansion, 

 diminishes the pressure upon the carbon disks, and thus pro- 

 duces an increase in their electrical resistance. This simple 

 apparatus thus supplies a means of regulating the strength 

 of small currents, so as to vary only within certain narrow 

 limits. 



According to Joule's law the heat generated in the strip 

 per unit of time depends upon its resistance, and upon the 

 square of the current ; or 



On the other hand, the dissipation of heat by radiation de- 

 pends upon the surface of the strip, and upon the difference 

 between its temperature and that of the air. Therefore, in 

 order that the current C may remain constant, it must, at 

 every moment, be equal to the square root of the tempera- 

 ture divided by the resistance ; and this function is performed 

 automatically by the regulator, which throws in or takes out 

 resistance in the manner described, according as the tempe- 

 rature increases or diminishes. 



The regulating instrument may also be adapted to the 

 measurement of powerful electric currents, by attaching to 

 the end of the sensitive strip a lever, with a pencil pressing 

 with its point upon a strip of paper drawn under it in a 

 parallel direction with the lever by means of clockwork, a 

 datum line being drawn on the strip by another pencil. The 

 length of the ordinate between the two lines depends, in the 

 first place, upon the current which passes at each moment, 

 and, in the second place, upon the loss of heat by radiation 

 from the strip. 



If B/ is the resistance and W the heat with a current C 

 and temperature T', then, by the law of Joule, 



and the loss by radiation is equal to 



in which T' is the temperature of the strip, T that of the at- 

 mosphere, and S the surface of the strip. 



