654: Mr. C. Gr. Barkla on the Velocity of 



At the middle of the terminal bridge KK/ was a loop of 

 one turn, in which was fixed a glass tube closed at one end. 

 The detector-needle consisted of a bundle of fine iron wires, 

 fixed by paraffin-wax in the end of a glass tube, which was 

 just wide enough to slide inside the fixed tube. 



Fig-. 1. 



B' 



6' 



In order to compare the intensities of the oscillations at 

 the loop under different conditions, it was of course necessary 

 to have the detector-needle in exactly the same position for 

 each observation. Accordingly, the inner tube was pushed 

 home in the fixed tube (in which position the loop passed 

 round the middle of the detector-needle), and turned round 

 its own axis till a fine pointer attached to the outer end came 

 into a position indicated on the stand. 



To obtain a measure o£ the intensity of the oscillations at 

 R during the different observations, the needle was first 

 magnetized to saturation. When placed in a given position 

 near the magnetometer, the deflexion of the magnetometer- 

 needle was not absolutely the same after each magnetizing 

 process, so that for the earliest observations the detector 

 needle was frequently demagnetized and remagnetized again, 

 to make this initial reading as nearly as possible a constant 

 quantity. This, however, was found to be unnecessary, as 

 the final value of the deflexion, after the occurrence of the 

 oscillations whose intensity the detector indicated, appeared 

 to be independent of these slight initial variations of the 

 saturation reading (see Table I.). 



When saturated, the needle was placed in position at R, 

 and sparking between the knobs N and N' was set tap for a 

 fixed period, by an induction-coil whose terminals were 

 connected to the large plates. 



After the demagnetizing process, the detector-needle was 

 placed in a definite position near the magnetometer, and the 



