8 
MR, E. RUTHERFORD ON A A[AGNETTC DETECTOR OF 
In practice copper wires were soldered on to the extremities of the steel needle, 
which is placed in position before a magnetometer. A magnetizing solenoid is 
wound ovei’ the needle, and after every experiment a steady current vras sent 
through in order to re-saturate the needle. 
Both the “longitudinal” and “ solenoidal” detectors may be very readily used to 
compare the intensities of currents in multijole circuits when the period of oscillation 
is the same for each. The best form of the solenoidal detector is explained later, 
and it has the advantage of being able to distinguish between the intensity of the 
first and second half oscillations. 
Detection of Waves in F'vee Space. 
It has been shown tha.t the amount of demagnetization of a magnetized needle 
depends on the fineness of the wire and the number of turns per centim. on the 
mEignetizing solenoid. 
If a short piece of thin magnetized steel wire be taken, and a large number of 
turns wound over it, it is a very sensitive means of detecting electrical oscillations 
in a conductor when the amplitude of the oscillations is extremely small. It was on 
this principle that a detector for electrical waves was devised, which proved to be a 
sensitive means of detecting Hertzian waves at considerable distances from the 
vibrator. 
About twenty pieces of fine steel wire ’007 centim. in diameter, each about 
1 centim. long, and Insulated from each other by shellac varnish, formed the detector 
needle. A fine wire solenoid was wound directly over it, of two layers corresponding 
to about 80 turns per centim. As the solenoid w^as of very small diameter, about 
15 centim. of wire served to wind the coil. This small detector wns fixed at the 
end of a glass tube, which was itself fixed on to a wooden base, the terminals of the 
detector coil being brought out to mercury cups. 
Fig. 2. 
-io qJ- 
C A B D 
S (fig. 2) represents the detector needle and the solenoid wound over it. A and B 
are the mercury cups. CA and BD wmre twm straight rods wdiich served as receivers, 
one end of each being placed in the mercury cups. 
The detector needle was strongly magnetized and placed before a small magneto¬ 
meter, the deflection due to the needle being compensated by an auxiliary magnet. 
If the receiving wires were parallel to the electric force of the wave from the 
