15 
1 908—9.] Vibrational Neutral Points in Magnetised Iron. 
as anticipated, much less than with mechanical vibrations. Lengths of 
iron wires 100 cms. long and 0*028 cms. diameter, annealed and varnished, 
were therefore made up into bundles of five wires each. Four of these 
bundles could be inserted into either of two narrow glass tubes 41 cms. 
long. Round the first, five silk-covered copper wires 0*056 cms. diameter 
were closely wound in a single layer, through which oscillations could be 
passed in parallel. This formed oscillation solenoid No. 1, with three 
(15/5) effective turns of wire per cm. of length. The second glass tube 
was wound with a single silk-covered copper wire of the same diameter, 
also in a single layer throughout its entire length, through which oscilla- 
tions could be passed in series. This second glass tube formed oscillation 
solenoid No. 2 with fifteen (effective) turns per cm. of length. 
Either of these solenoids, after insertion in the magnetising coil at 
right angles to the earth’s field and as previously described, could be con- 
nected with the secondary of an air oscillation transformer consisting of 
eights turns of copper wire 0*264 cms. diameter, insulated from its supports 
by means of india-rubber tubing. The radius of the secondary was 8*7 
cms., and it was earthed at a point approximately equidistant from its 
terminals.* The primary, consisting of one turn of copper wire insulated 
in the same way, 0*264 cms. diameter and 15 cms. radius, was connected 
with the terminals of a small Wimshurst machine (8-inch plates) with one of 
its small Leyden jars in series. A row of spark gaps, each 1 mm. in 
length, formed a shunt across the terminals of the Wimshurst, and any 
number from one to ten could be introduced into this parallel circuit by 
means of a sliding wire connection, pointed at one of its ends so as to rest 
securely in small holes drilled in the spark balls. These were of brass, 1*2 
cms. diameter, and only approximately spherical. They were cemented to 
vertical glass supports. The above description is illustrated in fig. V., 
which may be regarded as a plan of the apparatus drawn to scale. In the 
preliminary experiments with the single steel wire (*092 cms. diameter) 
a single micrometer spark gap was used. It could be increased in length 
from 1 to 12 mm. 
Experimental Methods. 
Cyclic Fields. — The sequence of operations observed and the galvano- 
meter readings taken were entirely similar to those which have been fully 
described for mechanical vibrations, with the following exception. It was 
* This was found to be absolutely necessary. Without the earth connection, the spot of 
light moved over a large part of the scale, more especially just before the passage of the 
spark ; with the earth connection, a sharp deflection was obtained on the passage of the 
spark, in the vast majority of cases from a steady zero position. 
