204 THE SCIENTIFIC PAPERS OF 



extent the resistance of the regulating rheostat. The resistance 

 coils, by which adjoining contact springs are connected, may be 

 readily changed, so as to suit particular cases ; they are made by 

 preference of naked wire, in order to expose the entire surface to 

 the cooling action of the atmosphere. 



In dealing with feeble currents, I use another form of regulator, 

 in which disks of carbon are substituted for the wire rheostat. 

 The Count du Moncel, in 1856, first called attention to, and 

 Mr. Edison more recently took advantage of, the interesting 

 circumstance that the electrical resistance of carbon varies in- 

 versely with the pressure to which it is subjected, and by piling 

 several disks of carbon one upon another in a vertical glass tube, 

 a rheostat may be constructed which varies between wide limits, 

 according as the mechanical pressure in the line of the axis is 

 increased or diminished. Fig. 4, Plate 19, represents the current 

 regulator based upon this principle, and the foot-notes below the 

 figure furnish the explanation of parts. A steel wire of say 0'3 

 milim. diameter is drawn tight between the end of a bell-crank 

 lever (L) and an adjusting screw (B), the pressure of the lever 

 being resisted by a pile of carbon disks (C) placed in a vertical 

 glass tube. The current passing through the steel wire, through 

 the bell-crank lever, and through the carbon disks, encounters the 

 minimum resistance in the latter so long as the tension of the 

 wire is at its maximum ; whereas the least increase in temperature 

 of the steel wire by the passage of the current causes a decrease 

 of pressure upon the pile of carbon disks, and an increase in their 

 electrical resistance ; it will thus be readily seen that, by means of 

 this simple apparatus, the strength of small currents may be 

 regulated so as to vary only within certain narrow limits. 



The apparatus described in Figs. 1 to 3, Plate 17, may be 

 adapted also for the measurement of powerful electric currents : 

 an application which is represented by Figs. 5 and 6, Plate 18. 

 The variable rheostat is in this case dispensed with, and the lever 

 (L) carries at its end a pencil (P) pressing with its point upon a 

 strip of paper drawn under it in a parallel direction with the lever 

 by means of clockwork. A second fixed pencil (D) draws a 

 second or datum line upon the strip, so adjusted that the lines 

 drawn by the two pencils coincide when no current is passing 

 through the sensitive strip. The passage of a current through 



