INDUCTANCE AND CAPACITY 



373 



begin to cause irregularities. The results are independent of 

 the period of the galvanometer. 



i 2 



Space S in Terms of Number of Turna of Screw Q 



FIG. 220. Illustrating results attained by use of Zeleny discharge key. 



Deflection Method Using a Commutator. By means of a 

 rotary commutator the condenser may be rapidly charged and 

 discharged, possibly 100 times a second. The galvanometer will 

 then take up a steady deflection, and after the instrument has 

 been properly calibrated the capacity may be calculated. This 

 method is adapted to the measurement of small capacities which 

 are without absorption and leakage; for example, air condensers, 

 or the capacities between wires arranged as in a transmission 

 line. 



The commutator by which the charging and discharging are 

 effected should be substantial in design and directly driven by a 

 motor of considerable power which is provided with a flywheel 

 and supplied with current at a fixed voltage so that the number 

 of discharges per second may be constant. Fig. 221 shows the 

 device as used by Fleming and Clinton. 10 



In order to obtain good contacts copper brushes should be 

 employed. It is essential that surface leakage at the commu- 

 tator be eliminated. For this reason the brushes as they pass 

 from one active segment to the next must not be supported by 

 substances like mica or agate, for after a time these become cov- 

 ered with a conducting coating. Air must be used as the insula- 

 tion between the segments. 



In the form of commutator shown in Fig. 221, A and B are two 

 crown wheels of composition, about 4 in. in diameter. They are 



