538 
PROFESSOR J. A. EWING ON EXPERIMENTAL 
continuously by means of a “ slide ” of very simple construction, which is shown in 
the annexed figure. This consisted of a tall cylindrical glass jar filled with a dilute 
solution of sulphate of zinc, with a block of amalgamated zinc fixed at the top, and 
another at the bottom, the blocks being connected by the terminals 1 and 3 to the two 
poles of the battery. A third block of zinc was raised or lowered between these, 
through the fluid, by means of a cord and pulley, and formed the sliding terminal 
of the magnetising solenoid, of which one end was connected to it by the terminal 2, 
and the other end was connected through a commutator to one of the fixed blocks. 
This liquid slide gave a remarkably effective and convenient means of varying the 
magnetising current, and no difficulty was experienced in keeping the current 
constant when it was desired to do so. 
The position of the magnetised wire with respect to the magnetometer was such 
that any change in distribution of magnetism produced minimum effect, the displace¬ 
ment of the upper “ pole ” being at right angles to the line joining it with the 
magnetometer, while the lower pole was comparatively inoperative. The reduction of 
the observations to absolute measure presents no novel features. 3 was first evalu¬ 
ated, the deflection due to the magnetising solenoid alone being separately determined, 
and subtracted from the observed magnetometer readings before these were reduced 
to find 3- 33, where it is given, was afterwards calculated from 3 and <§• 
Let PP' be the magnetised wire, whose length is l and moment hn. Let M be 
the position of the magnetometer, distant r from P and r from P', and let H be the 
horizontal component of the earth’s field, acting perpendicular to the plane MPP 7 . 
Then if 6 be the deflection of the magnetometer needle, 
H sin 6= 
cos 0, 
Hr 2 tan 6 
Or, calling d the diameter of the wire, 
3 : 
Aim 
7 TCP 
4/Iir 2 tan 0 
