being just that between the insides of

the block ends. The sufficiently long rod of magnetic material to
be tested, having its lower end faced cpiite true, passes freely but
closely through the top end of the block, down through the solenoid,
and beds on the carefully " faced " inside of the bottom end of the
block. The protruding end of the rod has a metal pin through
it, which is caught by a double hook on the lower end of an ordinary
spring balance, the top end of which is suspended by a gut cord
passing over a fixed pulley and attached to the lever show^n. This
permeameter is also fitted ^^■ith an arrangement for testing the




358



Apparatus.
specimen ballistically. It consists of a small flat coil fixed to a brass
plate, which slides backwards and forwards between guides. The rod
passes through this coil and beds as before on the block, at the same
time keeping the coil back against the force of two spring strips on the
outside of the block. Immediately the rod is suddenly pulled up the
coil flies out, and a circuit joined to its terminals will receive an electro-
magnetic impulse proportional to the field just broken. In this way
the ballistic and traction methods can be made to check one another
in the final results obtained.
Standard Magneto = Inductor.
