468 



Intelligence and Miscellaneous Articles. 



produce equilibrium in the bridge — that is, will be such as to bring 

 the deflection of the galvanometer to the zero of the scale. We 

 shall then have the strength of the induced currents expressed as 

 resistances. One advantage of this method is this — that the read- 

 ings are always reduced to the same point, the zero of the scale. 

 This method also has a wider range than that of merely taking the 

 swing of the galvanometer-needle ; for currents which would throw 

 the spot of light in a reflecting galvanometer off the scale can be 

 readily kept on the scale in this method by merely altering the 

 ratio of E 3 to R 4 in the "Wheats tone's bridge. Since the shunting 

 of induced currents is accompanied with difficulties, this method is 

 especially advantageous. When the bridge was set up so that the 

 smallest variation in the resistance of the branch containing the 

 inductorium gave the greatest variation in the current going through 



the galvanometer, namely when — ° =0, S being the current 



dR 3 



through the galvanometer, and the resulting value of E 3 being 

 K 3 =\ / — £ — — t — l?, in which Gr is the resistance of the galva- 



V vx -j- 2x1^ 



nometer, B that of the circuit exterior to the Wheatstone's bridge, 

 it was found that the induced currents could be measured to one 

 hundred-thousandth of an ohm. 



The following Table contains a comparison of the induced cur- 

 rents produced by making and breaking the circuit. The first two 

 columns contain the variation in ohms of the variable resistance of 

 the bridge ; the third and fourth columns give the strength of the 

 induced currents on making and breaking, expressed in ohms. 



Change in 



Change in 



Strength of 



Strength of 



the resistance 



the resistance 



Induced Current 



Induced Current 



on breaking. 



on making. 



on breaking. 



on making. 



650 



600 



•00325 



•00300 



700 



680 



•00350 



•00340 



720 



720 



•00360 



•00360 



720 



750 



•00360 



•00375 



700 



700 



•00350 



•00350 



850 



850 



•00425 



•00425 



Care should be taken to send the induced currents to be com- 

 pared in the same direction by means of a pole-changer. It will be 

 seen from the above Table that the equality of the currents on 

 making and breaking can readily be proved by this method. — Silli- 

 man's American Journal, May 1873. 



