BLECTRTCAl. WAVES AND SOME OF ITS APPLICATIONS. 
II 
wire round the detector needle. Since tlie reduction of magnetic moment was nearly 
proportional to the amplitude of the current, the intensity of currents at A^arious 
points could be very approximately compared. 
Fig. 3. 
The ordinary Hertz arrangement (fig. 3) was set up for obtaining free vibrating 
circuits. 
A and B were two plates set up vertically. Beside them were two small plates 
A' and B', and long wires A'E, B'F were led from these plates. A fixed bridge was 
jjlaced at EF, the ends of the wires, and a detector placed at the middle point of 
EE, with a small magnetometer fixed in position. 
A sliding bridge, CD, was then moved till the fall of the deflection of the detector 
needle was a maximum. This position of the bridge could be very accurately 
determined, for a movement of the bridge through 1 centim, altered the deflection 
considerably. The detector was then placed in various parts of the circuit CE, and 
the amplitude of the current at the different points determined. It was found that 
the current was a maximum at C and the middle point of EF. A well-defined node 
was found at the middle point of CE. 
The length CEFD was thus half a AvaAm-length, 
Since the use of the bolometer has been the only means of accurately investigating 
waves along wires, it was interesting to observe whether the magnetic detectors 
were of the same order of sensitiveness as the bolometer. 
One turn of wire was wound round each of two glass tubes, sliding along the wires 
CE and DF, as in Reuben’s experiments. Instead of the fine bolometer wire, a 
detector needle, with several turns of wire around it, was placed in series Avith the 
two turns of wire. The charging and discharging of the small condensers, formed by 
the straight wires and the small coils around them, Avas quite sufficient to almost 
completely demagnetize the needle. By this method the movement of extremely 
small quantities of electricity could be detected and the sensitiveness Avas quite 
comparable with that of a delicate bolometer. 
No appreciable damping could be detected for the long wire circuits, showing that 
they Avere probably vibrating almost independently of the primary vibrator. 
If a metre or two of Avire was fixed to the pole of a Wimshurst machine, on the 
passage of a spark, there Avas evidence of a rapidly-oscillating current set up in the 
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