![Thumbnail: [Volume 27, 1894 page 494]](/journals/rsnz/images/rsnz_27/thumbnails/rsnz_27_00_0519_0494_ac_02.gif)
III. Effect Of Induced Secondary Currents On the Magnetization of Iron.
Plate XLIX., Fig. 10.
A primary coil of ten turns of insulated wire was placed inside a glass tube, and a secondary of twelve turns outside the tube. A discharge was passed through the primary, and induced currents obtained in the secondary, giving a small spark at B. When a magnetized steel needle 0.032 in diameter was placed in a solenoid of three turns to the centimetre the deflection fell from 300 to 261.
In order to see the effect of resistance on the amount of current in the secondary, 10 yards of platinoid wire was placed in series with the secondary. Deflection fell from 300 to 285.
On removing platinoid wire and placing a copper wire of same length and section in its place very little difference in
![Thumbnail: [Volume 27, 1894 page 495]](/journals/rsnz/images/rsnz_27/thumbnails/rsnz_27_00_0520_0495_ac_02.gif)
the deflection was observed, although resistance of platinoid wire was 130 ohms and of the copper not more than 3 ohms. This shows that the quantity that flows in the secondary is practically independent of the resistance of the conductors.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
This agrees with the theory; for if M be the coefficient of mutual induction between the two coils, and I self-induction of secondary, then the quantity of electricity circulating in the secondary is given by M/(R2 + p2L2)4, and when p is large R may be neglected in comparison with pL.
If L = 3,000, and frequency 2,000,000,
p = 12 × 106 approximately,
pL = 36.109.
Therefore, in order that R2 may be comparable with p2L2, R should be 10 ohms or more.
In the experiments considered, a short carbon rod of 10 ohms introduced into the circuit did not alter the effect on the needle, showing that pL must have been considerably greater in the experiment than in the calculation above. On adding a solenoid of sixty turns in the secondary the fall of deflection was scarcely appreciable, showing that the quantity circulating in the secondary depended on the inductance and not on the resistance of the conductors within wide limits.