ELECTRIC UNITS OF CHARGE BY G. C. LAURENCE, B. SC. 185 



An Electrical Method of Measuring the Frequency of 

 Commutation Simultaneously with the Balancing 

 OF the Bridge 



A magnetic tuning fork was used as the commutator in 

 the Maxwell Bridge. Its frequency was measured simultane- 

 ously with the balancing of the bridge in the following manner. 

 The fork was driven by an electric magnet and a make-and- 

 break contact operated by a storage battery. This intermittent 

 direct current was passed in series through the current winding 

 of a wattmeter. The potential winding of this wattmeter was 

 connected to the output of a motor-alternator driven at a very 

 slightly different frequency. When alternator potential and 

 fork current were in phase the needle of the wattmeter deflected 

 in one direction and when out of phase in the opposite direction, 

 so that the needle moved back and forth indicating the beat- 

 between the two. The frequency of the beats was the diff"ers 

 ence between the frequency of the fork and of the alternator. 

 The frequency of the alternator was measured with a stop-clock 

 and a revolution counter over periods of about 15 minutes during 

 which time the bridge was balanced and at the same time another 

 observer counted the beats indicated by the wattmeter. The 

 sum(or diflference) of the two frequencies gave that of the fork 

 with an error less than .05%, the stop clock being checked be- 

 fore and after each measurement with a good v/atch. A direct 

 current ammeter and a minature lamp were also used as beat 

 indicators, being connected so that both fork and alternator 

 currents, superimposed on one another, flowed through them. 



An Electrical Method of Adjusting the Cylinders so that 



THEY ARE CoAXIAL 



A cylinder within another cylinder has a minimum capa- 

 city when it is exactly coaxial with the outside one. When the 

 bridge was roughly balanced, decreasing the capacity of the 

 condenser had the eflFect of moving the galvanometer reading to 

 the left. So the inner cylinders were adjusted by turning the 



