150 Mr. A. Gray on the Determination of the 
But the total horizontal force exerted by the pendulum? is 
W u <7 tan 6 1 + W^g tan 6. 2 , where Wj, W 2 are taken in grammes, 
and g, the acceleration produced by gravity, as approximately 
981 centim. per second per second. Hence we have 
I^^Witan^ v7 2 tan6> 2 ). ... (5) 
From this formula the values of I can be calculated from the 
values of the other quantities found by experiment. The 
following are the results of some actual experiments made in 
September last : — 
in grammes. 
YT,, 
in grammes. 
in C.G.S. 
I, 
in C.G.S. 
I/y*. 
09 
2210 
24556 
•132 
3410 
26590 
■140 
4170 
2978-j (?) 
■156 
4373 
28032 
172 
4864 
28283 
(1) 100 
(2) 
(3) 
W 
(5) 
100 
The same method was applied to find the value of I for 
different parts of the field on the two sides of the middle 
position. With two operators (one to move the pendulums, 
the other to watch the wire and take the galvanometer- 
readings) the experiments were quickly and satisfactorily 
performed. 
The following modification of the same method has been used 
for the determination of I for several recorder magnets, with 
results agreeingwell with thoseobtained (for other instruments ) 
by the first method described above. Ahorizontal bar, carrying 
a divided scale, was fixed parallel to the equilibrium position 
of the plane of the coil, in a vertical plane passing through 
the glass siphon a little beyond the point of the aluminium 
arm supporting it. A single block, to which the upper end 
of a pendulum-thread was attached, was made movable along 
the horizontal bar, so that any displacement of the point of 
suspension could be observed by means of the divided scale. 
The pendulum was made of a brass weight, generally of one 
gramme, as bob, hung by a very fine silk thread long enough 
to allow the bob to hang a few centimetres below the siphon. 
When the coil was in the position of equilibrium, the pen- 
dulum was moved until the thread just rested against the 
siphon without deflecting it, and the reading on the scale was 
noted. A current from one or two Daniell's cells was then 
sent through the coil so as to deflect it from the equilibrium 
* Tbis column is given, as y was also the magnetizing current. 
