THE MEASUREMENT OF RESISTANCE 227 



To compensate for the changes in the resistance of the coil 

 leads, due to temperature, compensating leads as nearly like the 

 coil leads as possible and in a similar environment are inserted 

 in the adjustable bridge arm, or three leads are used as shown at 

 the right hand of Fig. 127. 



RESISTIVITY, CONDUCTIVITY 



It is well known that the electrical resistances of conductors 

 having the same dimensions, but made of different materials, 

 will differ. Each metal has its characteristic resistance, which, 

 when determined for some unit specimen of the material and at a 

 stated temperature, gives the resistivity or specific resistance. 

 Various methods of expressing this property are used; the three 

 most commonly employed are shown below. 



1. The resistivity is frequently expressed as the resistance, 

 in ohms, or microhms, of a wire 1 cm. long and having a cross- 

 section of 1 sq. cm. This has commonly been called the centi- 

 meter cube resistivity. This name frequently gives rise to false 

 notions as to the relations of the quantities involved. This 

 quantity is better designated as the ohm (cm.) or microhm (cm.) 

 resistivity, which terms are descriptive of the units involved. 

 If it be represented by S A , then a wire L cm. long and A sq. cm. 

 in section will have a resistance 



* - ~f- (28) 



2. Another unit in common use is the resistance in ohms of a 

 wire 1 ft. long and 1 mil or 0.001 in. in diameter. This is com- 

 monly called the foot-mil resistivity. It should, however, be 

 designated as the ohms (mil, foot) resistivity. If it be repre- 

 sented by S D , then for circular wires, if the length L be expressed 

 in feet and the diameter D in mils, 



R = (29) 



3. The third method of expressing resistivity is in terms of the 

 resistance in ohms of a wire of uniform cross-section, 1 m. long 

 and 1 gm. in mass. This has commonly been called the meter- 

 gram resistivity. The designation ohm (meter, gram) resistivity 



