282 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



of the resistance is then found by dividing the electromotive force 

 by the current. 



158. Measurement of very high resistances. Insulation re- 

 sistance. Very large resistances cannot be easily measured by 

 the methods outlined above. Consider, for example, an insulated 

 cable consisting of a core of copper wire surrounded by a layer 

 of rubber and inclosed in a sheath of lead. If the lead sheath 

 is connected to one terminal of a battery and the copper core to 

 the other terminal, a certain amount of current will flow through 

 the insulating layer of rubber, that is to say, the rubber is not a 

 perfect insulator (infinite resistance). Very high resistances are 

 usually determined by measuring, with a sensitive galvanometer, 

 the current / which is forced through the given resistance by a 

 large known electromotive force E. Then according to Ohm's 

 Law the resistance is equal to E/f.* 



Example. One terminal of a i,ooo-volt battery is connected 

 through a very sensitive galvanometer to the outside tin-foil 

 coating on a glass jar, and the other terminal of the battery is 

 connected to the inside coating. The. current, as indicated by the 

 steady deflection of the galvanometer, is 1.4 x io~ 10 amperes. 

 The resistance of the glass between the coatings is therefore 

 equal to 7,100,000 megohms (one megohm is equal to 1,000,000 



ohms). 



MEASUREMENT OF ELECTROMOTIVE FORCE. 



159. The potentiometer. The potentiometer is a device which 

 is now extensively used for the accurate measurement of electro- 

 motive force. The essential features of this instrument may be 

 best described by referring to the slide-wire form f of the poten- 

 tiometer, the essential features of which are shown in Fig. 209. 



^Insulators do not conform to Ohm's Law, or, in other words, the current through 

 an insulator is not strictly proportional to the electromotive force. Different values 

 will therefore be obtained for the insulation resistance according to the value of elec- 

 tromotive force used. 



f Commercial forms of the potentiometer for accurate electromotive force measure- 

 ments are described in Practical Physics by Franklin, Crawford and MacNutt, Vol. 

 II, pages 66-74. 



