322 ELEMENTARY LESSONS ON [CHAP. vi. 



EXAMPLE. Suppose we had a wire, whose resistance we 



knew to be between 46 and 47 ohms, and wished to 



measure the fraction of an ohm, we should insert it at D, 



and make A 100 ohms and C 10 ohms ; in that case D 



would be balanced by a resistance in B 10 times as great 



as the wire D. If, on trial, this be found to be 464 ohms 



we know that D = 464 x io -~ 100 = 46*4 ohms. 



In practice the bridge is seldom or never made in the 



lozenge-shape of the diagrams. The resistance-box of 



Fig. 132 is, in itself, a complete "bridge," the appropriate 



connections being made by screws at various points. In 



using the bridge the battery circuit should always be 



completed by depressing the key K x before the key 



K 2 of the galvanometer circuit is depressed, in order 



to avoid the sudden violent " throw " of the galvanometer 



needle, which occurs on closing circuit in consequence of 



self-induction (Art. 404). 



36O. Measurement of Electromotive-Force. 

 There being no easy absolute method of measuring 

 electromotive-forces, they are usually measured relatively -, 

 by comparison with the electromotive-force of a standard 

 cell, such as that of Daniell (Art. 170), or better still 

 that of Latimer Clark (Art. 177). The methods of 

 comparison are various ; only three can here be men- 

 tioned. 



(a) Call E the electromotive-force of the battery to 

 be measured, and E' that of a standard battery. 

 Join E with a galvanometer, and let it produce 

 a deflection of ^ degrees through the resistances 

 of the circuit ; then add enough resistance r to 

 bring down the deflection to <5 2 degrees say io 

 degrees less than before. Now substitute the 

 standard battery in the circuit and adjust the 

 . resistances till the deflection is 6\ as before, and 

 then add enough resistance, / , to bring down the 

 deflection to d 2 . Then 



/ : r = E' : E, 

 since the resistances that will reduce the strength 



