200 



ELECTRICAL MEASUREMENTS 



Therefore M L and m L are made adjustable by including in each a 

 mercury slide resistance (a and b in Fig. 112). This consists of 

 an ebonite tube about 12 cm. long with a 3-mm. bore. The 

 terminals are at the upper and lower ends of the tube and an amal- 

 gamated copper plunger serves to displace and short-circuit 

 more or less of the mercury. This form of adjustable resistance 

 is remarkably definite in its action. 



To carry out the adjustment it is necessary to add two switches, 

 $1 and Sz, as shown in Fig. 112, by which the arms M c + N c 

 and m c + n c may be short-circuited. 



The final balance is attained by four steps: 



1. With both S l and S t open, 

 the bridge is balanced as usual 

 by adjusting the ratio arms. 



M ^ 2. The switch *Si is closed; 



tl*j balance will be upset; it 

 is restored by adjusting M L by 

 means of the rheostat a. This 

 makes 



FIG. 112. Wenner arrangement ,, Ar 



f 9 r eliminating the effect of lead re- ^i^c = NiMc, very closely. 

 sistances in the Thomson bridge. 



3. The switch Si is opened 



and S z is closed and the balance restored by adjusting the value 

 of m L by means of the rheostat 6. This makes 



m c n L = n c m L , very closely. 



4. With both Si and Sz open, the bridge is finally balanced 

 by adjusting the double ratio slides. If this last adjustment 

 requires a considerable change in the setting of the ratio slides, 

 the adjustments are repeated, 



Measurement of Resistances in Permanently Closed Circuits. 

 For a method of measuring a resistance which is included in a 

 circuit which cannot be opened, see page 96. 



MEASUREMENT OF HIGH AND OF INSULATION RESISTANCE 



The measurement of insulation resistance, using direct- 

 current potentials of a few hundred volts, is of great practical 

 importance because oMts utility as^a means of separating the 



PROPERTY OF ELECTRICAL LABORATORY, 



\ FACULTY OF APPLIED SCIENCE. 



