2 
]VIR. E. RUTHERFORD OH A MAGNETIC DETECTOR OF 
In 1842 Professor Henry was led to susp-ct from the anomalous magnetization of 
steel needles that the Leyden jar discharge was oscillatory. 
Professor Henry, Abria, and several others, used steel needles in their attempts 
to determine the direction of induced currents in secondary and tertiary circuits, 
when the Leyden jar was discharged through the primary, but with conflicting 
results. 
Lord Rayleigh (‘Phil. Mag.,’ vol. 39, 1870, p. 429) made use of steel needles in a 
magnetizing spiral in investigating the maximum current of a break for ordinary 
induction circuits. 
The general subject of the magnetization of iron, for rapid oscillatory currents, has 
been worked at by many different experimenters ; Lord Rayleigh, using oscillatory 
currents of a frequency up to 1050 a second, showed that iron wires showed consider¬ 
able increase of resistance, and deduced the value of the permeability of the wire. 
Trowbridge (‘Phil. Mag.,’ 1891) has shoAvn that iron wires rapidly damp down the 
oscillations of the Leyden jar discharge, and from his results deduced a rough vedue 
for the permeability of the specimens tested. 
' V. Bjerknes (‘Electrician,’ November 18, 1892) found that the damping out of 
oscillations in a Hertzian resonator takes place much more rapidly in a resonator 
of iron than when it is made of non-mag^netic material. 
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Magnetization of Iron hy a Leyden Jar discharge. 
If a piece of steel wire, several centimetres in length, be taken and placed in a 
solenoid of a few turns, on the passage of a discharge the ware will be found to be 
magnetized. The magnetization is, in general, small, and increases slightly in amount 
when a succession of discharges are passed in the same direction. 
Fig-1. 
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Fig. 1 shoAvs the arrangement. A and B are the poles of a Wimshurst machine 
or an induction coil, C the condenser, S the air-break, and D the solenoid in Avhich 
the steel wire is placed. The charging-up of the condenser before the spark passes 
was found to have no effect in mao'netizing the needle. 
In all experiments to follow, the magnetization of the needles was tested by means 
