212 THE SCIENTIFIC PAPERS OF 



the rheostat is short-circuited, and the regulator offers no other 

 resistance to the current than that of the horizontal strip itself. 

 In setting the regulator to work the regulating-screw (B) is drawn 

 on sufficiently to bring the whole of the contact-screws into 

 contact with the disk. The passage of the current through the 

 strip will have the effect of raising its temperature to an extent 

 commensurate with the electrical resistance ; and in the same 

 measure the strip itself will be elongated, and cause the spindle 

 with the contact-disk to descend. 



Another form of this instrument depends for its action upon the 

 circumstance discovered by the Count du Moncel in 1856, and 

 more recently taken advantage of by Mr. Edison, that the electri- 

 cal resistance of carbon varies inversely Avith the pressure to which 

 it is subjected. A steel wire of OU 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, 0, 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 

 temperature 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 ap- 

 paratus 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 depends 

 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 temperature 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 temperature increases or diminishes. 



