6 
ME. J. E. H. GORDON ON THE DETERMINATION OE 
To the end of the latter a magnet and mirror weighing only about a grain was hung 
by a silk fibre, so that when the pin was at the highest point the magnet hung in the 
axis of the cylinder. The helix being placed coaxial with the coils was pushed endways 
so that the centre point of the coils was just outside one end of it. 
The cylinder was then inserted and adjusted, by means of cross-wires, so that the 
mirror hung exactly at the centre point, and a mark was then made on the handle 
corresponding to a mark on the stand of the instrument, by reference to which the 
magnet could always be brought to the same position. 
Thus while the helix was slid along the axis, the magnet inside it could always be 
placed at the centre point of the dynamometer. The distance between the inside ends 
of the helix was divided into six equal parts by means of seven pins stuck as sights into 
a slip of wood fixed along the top edge. Thus by sliding the helix along till any one of 
the sights was between the vertical threads placed midway between the coils, the force 
at that point due to the helix could be compared with that due to the dynamo- 
meter by means of the needle at the centre. 
The comparison was made by sending currents of different intensities through the helix 
and coils in opposite directions, and varying them till there was no action on the needle. 
This was effected by dividing a current and passing one portion through the coils and 
the other through the helix, and interposing different resistances in each branch. 
The annexed diagram shows the connections. 
B is the battery, R and r the resistance- coils. 
D, the dynamometer-coils. 1 And let these letters also represent 
H, the helix. J their resistances. 
« the magnet and mirror, whose angular position is observed by 
S, the scale and lamp. 
K W are contact-keys. 
Now when the actions on the needle are equal, the powers of the coils are inversely 
as the currents in them, that is, directly as the resistances r+D and R+H. 
