1905.1 



Oscillations into Continuous Currents. 



479 



associated mirror galvanometer a means of detecting feeble alternating 

 electric currents or oscillations. Another method is to employ ' 

 differential galvanometer and two vacuum valves. These must then 

 be arranged, as shown in fig. 2, one circuit G! of the differential galvano- 

 meter is in series with one valve ^ and the other circuit G 2 with the other 

 valve V 2 , but so joined up that currents flowing through the valves in 

 opposite directions pass round the two galvanometer wires in the 

 same direction as regards the needle and, therefore, their effects are 

 added together on the galvanometer needle. Each valve must then 

 have its own separate insulated battery to ignite the filament. Also, 

 it is necessary that the connection with the oscillatory circuit must be 

 made in both cases to the hot filament by that terminal which is in 



FIG. 2. 



connection with the negative pole of the local battery used to ignite 

 the filament (see fig. 2). 



This arrangement of a differential galvanometer and two valves 

 transforms, of course, more of the alternating oscillation into direct 

 current than when one valve alone is used. It provides us with a 

 means of detecting electrical oscillations not merely in closed circuits 

 but in open electrical circuits. 



When so using it, it is necessary to associate with the oscillation 

 valve and galvanometer an oscillation transformer for raising the 

 voltage. The resistance of these valves, when in operation, may be 

 anything from a few hundred ohms up to some megohms, depending . 

 on the state of incandescence of the filament and upon the electro- 

 motive force employed to drive the current through the vacuous space, 

 as well as upon the size of the filament and the plate. This resistance 



VOL. LXXIV. 2 N 



