SCIENCE AND PRACTICE. 299 



the same kind, or a current will be set up between them 

 which will make the observed resistance smaller or greater, 

 according as the current due to the plates is in the same, or 

 in the opposite direction to the current of the measuring 

 battery. 



45. Insulation Resistance by Bridge. As the resistance of 

 insulation is generally very great, in comparison with that of 



T 



the conductor of a cable, the proportion has to be made as 



large as possible. The limit to which we may go, in this 

 direction, we have seen is when 

 r_ = 10,000 

 p 10 



and, as the greatest value of R is 1 0,000, cable resistances under 

 ten millions of units may be measured by this method. 



The whole force of the battery is introduced by inserting 

 the contact plug in hole z 3 of T. One end of the cable is 

 connected to L, the other insulated. The direction of the 

 current is determined by the position of the two plugs in 

 Ui. The circuits of the currents are obvious. 



When the cables are long, or badly insulated, this method 

 of measuring the resistance of their dielectrics is the best ; 

 but for short or well- insulated cables it would not be found 

 sufficiently delicate, and the method by comparison of 

 the deflections of the galvanometer needle is to be pre- 

 ferred. 



46. Insulation-resistance by Deflection of Galvanometer 

 Needle. The bridge system is interrupted for this measure- 

 ment, and the current of the whole battery allowed to go 

 in a simple circuit through the galvanometer coils, and 

 into the cable. The resistances r and R are made infinite, 

 and o null, as in Fig. 142, by which the current of the 

 battery goes from the key k, through c, p, U 2 , coil of galva- 

 nometer, the other side of u 2 , a, L' L, cable, through the in- 

 sulating material to earth, and from earth through u x , battery, 

 u ly to key. During the whole time of measuring insulation 

 resistance, the beam of the key is depressed upon the work- 

 ing contact. The plugs of u l are placed so as to put the 



