INDUCTANCE AND CAPACITY 



423 



This arrangement gives a sharp tone which enables the null 

 point to be accurately determined. The frequency, however, is 

 somewhat uncertain and is controlled by the battery strength and 

 the distance between the transmitter and receiver diaphragms. 

 This simple arrangement has been improved by A. Campbell so 

 that a constant frequency may be obtained; this apparatus is 

 shown in Fig. 248. 



FIG. 248A. 



Microphone 



To Bridge 



FIG. 248. Campbell microphone hummer. 



The receiver diaphragm is replaced by a bar of mild steel 2.5 

 cm. in diameter and 23.7 cm. long (for a frequency of 2,000). 

 The bar is supported at its nodal points on two adjustable bridge 

 pieces. The frequency may be altered by changing the bar and 

 at the same time altering the capacity of the condenser so that 

 800, 1,000 or 2,000 periods per second may be attained. The 

 hummer is started by lightly striking the bar and then adjusting, 

 by the milled head F, the height of the magnet E, which main- 

 tains the vibration. To prevent electrostatic disturbances, the 

 winding D is covered with an electrostatic shield which may be 

 connected to earth. 



When using the hummer for measuring small capacities or 



