346-350] Measurements 303 



The arrangement of conductors in parallel is therefore seen to offer the 

 same resistance to the current as a single conductor of resistance 



1 



The reciprocal of the resistance of a conductor is called the " conductivity " 

 of the conductor. The conductivity of the system of conductors arranged in 



parallel is -~- + -~- + . . . , and is therefore equal to the sum of the con- 

 -tii J&1 



ductivities of the separate conductors. Also we have seen that the current 

 divides itself between the different conductors in the ratio of their con- 

 ductivities. 



MEASUREMENTS. 

 The Measurement of Current. 



349. The instrument used for measuring the current passing in a circuit 

 at any given instant is called a galvanometer. The theory of this instrument 

 will be given in a later chapter (Chap. XIII.). 



For measuring the total quantity of electricity passing within a given 

 time an instrument called a voltameter is sometimes used. The current, 

 in passing through the voltameter, encounters a number of discontinuities 

 of potential in crossing which electrical energy becomes transformed into 

 chemical energy. Thus a voltameter is practically a voltaic cell run back- 

 wards. On measuring the amount of chemical energy which has been stored 

 in the voltameter, we obtain a measure of the total quantity of electricity 

 which has passed through the instrument. 



The Measurement of Resistance. 



350. The Resistance Box. A resistance box is a piece of apparatus 

 which consists essentially of a collection of coils of wire of known resistances, 

 arranged so that any combination of these coils can be arranged in series. 

 The most usual arrangement is one in which the two extremities of each 

 coil are brought to the upper surface of the box, and are there connected 

 to a thick band of copper which runs over the surface of the box. This 

 band of copper is continuous, except between the two terminals of each coil, 

 and in these places the copper is cut away in such a way that a copper plug 

 can be made to fit exactly into the gap, and so put the two sides of the gap 

 in electrical contact through the plug. The arrangement is shewn diagram- 

 matically in fig. 97. When the plug is inserted in any gap DE, the plug 

 and the coil beneath the gap DE form two conductors in parallel connecting 



