508 Transactions. — Miscellaneous. 



tudinal current on the magnetism of a needle, however, not 

 only gives a qualitative method of comparison, but also very 

 accurate quantitative results. 



In the earlier part of this paper it has been shown that a 

 magnetized needle is demagnetized considerably under the 

 influence of an oscillating discharge of a frequency of about 

 100,000,000, such as is obtained from Hertz's dumb-bell 

 oscillator. 



Plate XLIX., Fig. 16. 



A and B were two metal plates of small capacity, S the 

 spark-gap. A small steel needle, C D, diameter 0-01in., was 

 placed in the discharge circuit. After the passage of several 

 discharges the deflection of the needle fell from 300 to 250 : 

 this corresponds to a depth of penetration of the discharge 

 of about 0-00045in. Sending several discharges in the same 

 direction had the effect of decreasing the deflection till a 

 steady state was arrived at, exactly as in the case of the 

 demagnetization of iron needles in solenoids. 



From the magnitude of the effect we have been consider- 

 ing, it is evident that a thin magnetized needle is a very con- 

 venient galvanometer for the measurement of the intensity of 

 electric waves at different parts of a circuit vibrating freely. 



In order to see if this were the case, the experimental 

 arrangement of Hertz for showing the existence of waves 

 along wires was used. 



Plate XLIX., Fig. 17. 



The plate B was about 10cm. behind the plate A. A 

 wire was taken from B to C, about 5 metres long. 



If the end C were free and insulated, and the small 

 detector placed in series near the end, no effect on the needle 

 was produced. If a metal plate were fixed to the end C, there 

 was immediately a fall of deflection of 50 divisions when the 

 vibrator was set working. This shows that when the end is ■ 

 free the point C is a position of minimum current, and when 

 the plate is added the current is a maximum at C. On 

 leaving the end free, and moving the needle to different dis- 

 tances from C, the fall of deflection gradually increased, and 

 then diminished again. 



The experiment was not proceeded with, as the subject of 

 the distribution of waves along wires has been worked out 

 experimentally by several methods. It shows, however, that 

 a magnetized needle is a very convenient galvanometer for 

 oscillating circuits for frequencies up to 500,000,000 per 

 second. 



The effects on fine steel wire, whether placed in series or 

 in a solenoid of several turns, may be used as a means of 



