of Small Inductances and Capacities. 593 
carries a brush which makes contact with the copper spiral 
at one place. 



foes Se Seeel 
T 
A spring presses against one of the brass end plates of the 
cylinder and carries a terminal. The sliding bar can be 
moved along to any position, its setting being determined by 
a scale, and the amount of inductance included between the 
two terminals can therefore be varied. The sliding bar can 
be lifted and moved quickly from one position to another, or 
moved slowly by turning the cylinder, in which case a gradual 
variation of inductance takes place. 
The inductance of the whole spiral was measured by the 
telephone-buzzer-bridge method, and found to be 227,000 cms., 
and it was also measured for every 5 or 10 turns and found 
to be as shown in Table LV. 
The value of the inductance predetermined by the formula 
above is 246,000 cms. for 100 turns, thus showing about nine 
per cent. excess over the real value. The above-mentioned 
formula cannot of course be applied to the case of a spiral 
having such a small dimension ratio as 45:1. The scale of 
Phil. Mag. 8. 6. Vol. 7. No. 41. May 1904. 28 
