246 Mr. 0. Heaviside on an advantageous Method of using the 



within them, so that, if the currents in both the coils are 

 equal, the needle is unaffected. The introduction of resistance in 

 the circuit of one coil will not affect the balance, provided an equal 

 resistance is introduced in the circuit of the other coil. Hence, 

 if on one side we place a rheostat, and on the other an unknown 

 resistance, the latter may be determined by varying the resist- 

 ance of the rheostat until a balance is obtained. Fig. 1 is a 

 representation of this ar- Fig. 1. 



rangement. The current 

 from the battery, having 

 a resistance b and electro- 

 motive force E, divides at 

 the point a between the 

 coil g and resistance x, 

 and the coil g' and re- 

 sistance r. When r=x,~^ 

 the needle is unaffected. 



By the following me 

 thod of using the diffe- 

 rential galvanometer, a much greater accuracy is obtained when 

 the unknown resistance whose value has to be determined is 

 small. 



Instead of dividing the current from the battery E between 

 the two coils, I join up the coils, so that the same current passes 

 through both of them, and by reversing one of the coils, g' } pre- 

 vent the current from influencing the needle (see fig. 2). The 



Fig. 2. 



rheostat r is connected to the 



two ends of one coil, and the 



resistance to be measured, x, to 



the two ends of the other. It 



will easily be seen, without 



further explanation, that when 



r = x, the currents in g and g' 



are equal; but should r not 



equal x, there will be a greater 



current in one coil than in the 



other, and the needle will move 



in obedience to the difference 



of these currents. It then only remains for me to show what, 



and under what circumstances, advantages are obtained by this 



method. To do so, we have only to compare the expressions 



for the difference- currents in the two methods. 



By the first method the resistance external to the battery is 



(x+g){r+g) . 



x + r + 2g 



