ELECTRICAL WAVES AND SOME OE ITS APPLICATIONS. 
5 
needle has the demao’netizin£>’ force of the needle assisting it, while the return 
oscillation has it in opposition. The return oscillation will therefore not be able 
to comj)letely remagnetize the surface layer already affected, but a thin layer will be 
left in the interior. After the passage of the next oscihation another layer will 
be added in the same direction, and so on, till the final effect will be that the surface 
of the needle will be magnetized in the opposite direction to the interior. 
If strongly-magnetized needles of the same diameter, but of different lengths, are 
taken and j)laced in the same solenoid, it will be found that the reduction of 
magnetic moment of the needle, due to the discharge, is greater the shorter the 
needle. This effect is cine to the demagnetizing influence of the ends, which is 
greater the shorter the needle. 
It was also found that if successive discharges be passed, the reduction of deflec¬ 
tion gradually increases, till it reaches a steady state, so that the passage of any 
farther number of discharges has no apparent effect on the magnetism of the needle. 
The following table shows the effect of varying the length of the needle, the 
diameter being kept constant, and also the effect of successive dischai-ges in each 
case. 
Needle ’08 centim. in diameter; frequency about 3 millions. 
Number of 
discharges. 
10'5 centims. 
6'4 centims. 
3*2 centims. 
1‘6 centims. 
75 centim. 
r\ 
U 
2.50 
250 
250 
250 
250 
1 
204 
190 
166 
150 
114 
2 
199 
182 
155 
135 
88 
5 
195 
175 
138 
115 
64 
10 
190 
170 
130 
107 
57 
20 
189 
166 
125 
102 
54 
50 
188 
162 
120 
98 
50 
In the above table each of the needles was placed at such a distance from the 
magnetometer to give the same stead}' deflection of 250. The vertical columns 
show how the deflection fell after the passage of the different numbers of discharges. 
The vertical columns correspond to needles 10‘5, 6'4, 3'2, 1’6, 75 centim. respec¬ 
tively. For the needle 10'5 centims. long, the deflection fell from 250 to 188, while 
for the short needle, 75 centim. long, the deflection fell from 250 to 50, although 
all the other conditions were precisely the same for each. 
It will be observed from the above that the first discharge is mainly instrumental 
in reducing the deflection, and that after ten discharges have been j^assed, the 
deflection has nearly reaciied its final value. 
Whenever a magnetized needle is placed in a solenoid and a discharge passed, 
there is always a reduction of the magnetization, the amount depending, for any 
given size of needle, on the intensity of the magnetic force in the solenoid and on 
