INDUCTANCE AND CAPACITY 347 



ful range. For instance, in an instrument having a useful range 

 of from 125 to 1,225 microhenrys the scale is uniformly divided 

 between 325 and 1,025 microhenrys; outside of these limits the 

 length of the divisions gradually decreases, but there are no sud- 

 den changes. 



The uniform scale is attained by using link-shaped coils; the 

 proper proportions were determined experimentally and are 

 given below. 



Referring to Fig. 205 



r = mean radius of semicircular end of coil 



c = 0.78r 



d = 2.2r 

 R = 2.26r 

 ri = O.Glr 

 r 2 = 1.39r 



The net cross-section of the fixed and movable coils is a square 

 having a side c units long. 



STANDARDS OF CAPACITY 



As examples of primary standards of capacity, that is, conden- 

 sers whose capacities in electrostatic units are calculated from 

 their dimensions, those used by Rosa and Dorsey 4 in their de- 

 termination of v, the ratio of the electromagnetic to the elec- 

 trostatic unit of quantity, may be taken. It is in connection 

 with the determination of v that the possible sources of error 

 in such primary standards have been most carefully studied. 



In order to be able to calculate the capacity of a condenser 

 with a high degree of accuracy, the dielectric coefficient of the 

 medium between the plates must be definitely known and the 

 medium must be free from absorption and from dielectric losses. 

 For these reasons air is always used as the dielectric in primary 

 condensers. Also, the distance between the plates must be so 

 great that the thickness of the dielectric may be determined with 

 accuracy. In consequence of these facts the capacities of primary 

 condensers are very small. 



Three forms of primary condensers have been developed, viz., 

 those with spherical, cylindrical and plate electrodes. A section 



