Galvanometer for Alternate Current Circuits. 319 



Other Uses of the Instrument. 



The voltmeter tests previously described show that the 

 instrument can be used to measure very small capacities, 

 especially in cases where it is possible to apply high voltages 

 to the condenser. It will be apparent from expression (2) 

 that if the voltage Y 1 applied to the condenser is in phase 

 with the voltage V applied to the field, and a known multiple 

 of it, the sensitiveness can be indefinitely increased by making 

 Vj large. 



Thus two circular brass plates of 7*3 cm. diameter and 

 about 6 mm. apart were tested as a condenser, and found to 

 have a capacity of (> x 10 " 6 microfarad. This capacity 

 caused a deflexion of 180 mm. when 102*4 volts were applied 

 to the 4000 turn coil, and, by means of a transformer, 

 81)0 volts were applied to the condenser plates through the 

 moving coil. The capacity tested being so small it was found 

 necessary to eliminate capacity effects associated with the 

 wires used for the connexions. This was done by taking the 

 difference of two deflexions obtained with the connexion to 

 one of the plates alternately made and broken. Earthing- 

 conditions had to be carefully attended to. Good values 

 have been obtained for the specific inductive capacities of 

 plates of various dielectrics, but we have as yet not had time 

 to properly carry out such tests, which for accurate results 

 require balance methods with guard-ring condensers. 



The instrument has not yet been tested with alternate 

 currents of higher frequencies than 100 cycles per second, 

 but there appears no reason to suppose there will be any 

 difficulty in the way of its use for high frequency work. 



Added July, 1910. — Mr. A. Campbell has drawn our 

 attention to a paper, previously unknown to us, in which 

 Stroud and Oates (Phil. Mag. 1903) describe an instrument 

 resembling in some respects the galvanoneter here referred to. 

 The paper gives data of the electromagmet showing that it 

 contained a greater volume of iron than that of the instru- 

 ment here described. But no details are given of the air-gap 

 between the poles, and it does not appear that any attempt 

 was made to produce a magnetic field whose phase is essenti- 

 ally in quadrature with that of the applied voltage. The 

 characteristics of the present instrument are the result of 

 such a relationship. In the Stroud galvanometer the field in 

 the o-ap is probably stronger and less uniform than that of 

 the instrument here described ; since the disturbing influence 

 of this field on the moving system seems much more serious. 



