THE MEASUREMENT OF RESISTANCE 



201 



good from the defective insulated wires during the process of 

 manufacture. Also, specifications as to insulation resistance as 

 measured by direct currents are inserted in contracts. 



It is to be understood that the results of this test are not 

 " resistances" in the same sense as those obtained for metallic 

 conductors by use of the Wheatstone bridge. As the test is 

 ordinarily carried out, the results give no means of calculating 

 the current which will finally flow through the insulation of the 

 wire under the prolonged application of a direct-current electro- 

 motive force (see page 207, Absorption Effects). It is to be 

 understood that the current which will flow through the di- 



FKJ. 113. Connection for measuring insulation resistance. 



electric when the applied electromotive force is periodic, es- 

 pecially at the high frequencies employed in telephony, involves 

 a very different " resistance" from that determined by this 

 method, at a given voltage it is distinctly lower, due to the energy 

 dissipated in the dielectric. 



Direct-deflection Method. Insulation resistances have very 

 high values and may be several hundred or several thousand 

 megohms, a megohm being 1,000,000 ohms. The method 

 usually employed in these measurements is really one of sub- 

 stitution; the necessary apparatus is shown in Fig. 113. 



The galvanometer G should be of the D'Arsonval type and 

 very sensitive; an instrument having a sensitivity of about 



