316 ELEMENTARY LESSONS ON [CHAP. vi. 



LESSON XXX. Electrical Measurements. 



355. The practical electrician has to measure electri- 

 cal resistances, electromotive -forces, and the capacities 

 of condensers. Each of these several quantities is 

 measured by comparison with ascertained standards, the 

 particular methods of comparison varying, however, to 

 meet the circumstances of the case. Only a few simple 

 cases can be here explained. 



356. Measurement of Resistance. Resistance 

 is that which stops the flow of electricity. Ohm's law 

 shows us that the strength of a current due to an electro- 

 motive force falls off in proportion as the resistance , in 

 the circuit increases. 



(a) It is therefore possible to compare two resistances 

 with one another by finding out in what proportion each 

 of them will cause the current of a constant battery to 

 fall off. ' Thus, suppose in Fig. 128 we have a standard 

 battery of a few Daniell's cells, joined up in circuit with 

 a wire of an unknown resistance R, and with a galvan- 

 ometer, we shall obtain a current of a certain strength, 

 as indicated by the galvanometer needle experiencing a 

 certain deflection. If we remove the wire R, and sub- 

 stitute in its place in the circuit wires whose resistances 

 we know, we may, by trying, find one which, when inter- 

 posed in the path of the current, gives the same deflection 

 on the galvanometer. Hence we shall know that this 

 wire and the one we called R offer equal resistance to 

 the current. Such a process of comparison, which we 

 may call a method of substitution of equivalent resistances, 

 was further developed by Wheatstone, Jacobi, and others, 

 when they proposed to employ as a standard resistance 

 a long thin wire coiled upon a wooden cylinder, so that 

 any desired length of the standard wire might be thrown 

 into the circuit by unwinding the proper number of turns 

 of wire off the cylinder, or by making contact at some 

 point at any desired distance from the end of the wire. 



