646 Prof. Pollock: Comparison of Periods of 
Eaperimental Details. 
Three methods have been used in this investigation. In the 
first, a condenser C, witha discharge circuit a a, is arranged as 
indicated in fig. 4, the diagram not being drawn to scale. 
Fie. 4. 
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a b © | If i | 
Fee RE 
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The condenser-plates are attached to wooden stands, one of 
which can be moved by a screw, the two condenser-plates 
always being strictly parallel. One of the discharge-wires 
is cut and ‘connexion remade through a pool of mercury. 
This enables the condenser-plates to be set at any distance 
from each other without disturbing the spark-gap s. Two 
small hollow brass boxes, b 0, are placed close to the condenser- 
plates. The boxes are provided with pegs which can be 
turned from the outside. To these pegs parallel wires are 
attached, the wires coming out through holes in the sides of 
the boxes. This arrangement of wires leading into hollow 
boxes was used by St. John *, and it enables the length of the 
wires to be conveniently altered without changing the 
capacity of the system near the condenser-plates. 
The wires are bridged at their ends and at some other 
point as at e. At a distance of 1 cm. from the terminal 
bridge is placed a third circuit f, whose vibration period it is 
desired to compare with that of the rectangle formed by the 
parallel wires and the two bridges. In fig. 4 the position of 
various third circuits used is shown in dotted lines. 
A preliminary investigation has to be made to determine 
the relation between the length of the wires ¢¢ and the dis- 
tance between the condenser-plates when the wire system and 
the condenser circuit are in tune. The procedure is as 
follows:—With a given length of the wires, observations are 
taken of the waves along them, due to a discharge of the 
* St. John, Phil. Mag. vol. xxxviil. (1894). 
er + 
