1879.] 



Measuring and Regulating Electric Currents. 



295 



into circuit : the result being that the temperature of the strip varies 

 only between very narrow limits, and that the current itself is ren- 

 dered very uniform, notwithstanding considerable variation in its 

 force, or in the resistance of the lamp, or other extraneous resistance 

 which it is intended to regulate. 



It might appear at first sight that, in dealing with powerful cur- 

 rents, the breaking of contacts would cause serious inconvenience in 

 consequence of the discharge of extra current between the points of 

 contact. But no such discharges of any importance actually take 

 place, because the metallic continuity of the circuit is never broken, 

 and each contact serves only to diminish to some 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 elec- 

 trical resistance of carbon varies inversely 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 5, represents the current regu- 

 lator based upon this principle, and the foot-uotes 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 curreut 

 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 4, may be adapted 

 also for the measurement of powerful electric currents — an application 

 which is represented by figs. 5 and 6, Plate 5. 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 



