ELECTRICAL WAVES AND SOME OF ITS APPLICATIONS. 
9 
vibrator, oscillations were set up in the receiver circuit, the surface layers of the 
needles were demagnetized, and there resulted a corresponding deflection of the 
magnetometer needle. 
The amount of the deflection, of course, depended on the amplitude of the 
oscillations set up in the receiver, and, therefore, on the distance from the vibrator. 
Long Distance Ex 2 :)eriments. 
When a Hertzian vibrator was used with plates 40 centims. square, and a short 
discharge circuit, quite a large deflection was obtained at a distance of 40 yards, the 
waves passing through several thick walls between the vibrator and receiver. 
Further experiments were made to see how far from the vibrator electromagnetic 
radiation could be detected. 
For the long distance experiments, the vibrator consisted of two zinc plates, 
6 feet by 3 feet, and separated by a short discharge circuit of about 30 centims. 
When large plates were used, a Wimshurst machine was equally efficient as a 
Ruhmkorff coil for exciting the vibrations. 
The first experiments were made over Jesus Common, Cambridge, the receiver 
being placed in one of the buildings on Park Parade. Quite a large effect was 
obtained at a distance of a quarter of a mile from the vibrator, and from the deflection 
obtained it was probable that an effect would have been got for several times that 
distance. 
When the vibrator was set up in the top floor of the Cavendish Laboratory, a 
small, but quite marked effect was obtained at Park Parade, a distance of over half a 
mile in the direct line. 
In this case the waves, before they reached the receiver, must have passed through 
several brick and stone walls, and many large blocks of buildings intervened between 
the vibrator and receiver. The length of wave given out by the vibrator was 
probably six or seven metres, and a wave of that length seemed to suffer very little 
loss of intensity in passing through ordinary brick walls. 
From an experiment tried in the Cavendish Laboratory it was found that the 
effect of six solid walls and other obstacles between the vibrator and receiver did not 
diminish the effect appreciably. When the vibrator was working in the upper part 
of the Laboratory, a large effect could be obtained all over the building, notwith¬ 
standing the floors and walls intervening. 
A large number of experiments were made on the effect of varying the length and 
diameter of the receiving wires. If a fairly dead-beat vibrator were used, e.g., plates 
with a short inductance, it was found that the deflection gradually increased with 
increase of length of the receiving wires, reaching a maximum which was unchanged 
by any further increase of length. 
MLCCCXCVII.—A. C 
