595 
of Edinlmrgli, Session 1883-84. 
edge of a wooden or brass ring of suitable thickness and diameter. 
Short lengths of wire at both ends of the copper spiral are left 
straight. These, after being well insulated, are twisted together and 
led to two terminals, which serve as electrodes. The ring containing 
the spiral is fixed on a base board with its plane vertical, and at 
right angles to the magnetic meridian, when the instrument is in use. 
A short magnet, rigidly attached at right angles to the lower end of 
a stiff wire, is suspended from a silk fibre, so that its centre is 
in the circular centre line of the anchor ring. ?Tear the upper end 
of the wire a long glass fibre 
pointer is attached, which 
moves over a horizontal disc 
graduated to degrees, and 
the whole is so enclosed so 
that the magnet, fibre, and 
pointer are free from currents 
of air. rig. 1 gives a sketch 
of the arrangement, showing, 
however, the convolutions of 
the wire much too far apart. 
These are in reality quite 
close together on the inner side, the spiral being tied tight into the 
rectangular groove by means of a cord. 
Let ?z = the number of convolutions in the spiral, 
r = radius of circular axis of coil = OB, 
C = current strength ; 
then, if 6 be the deflection, and H « horizontal intensity of the 
earth’s magnetism, we have 
'2nC ^ . 
cos 6^ = H sin 0 ^ 
r 
C = — tan 6 
2n 
From this formula it will be seen that the galvanometer constant 
can be very easily determined, since it depends only upon r and n. 
Since the endless coil, when carrying a current, forms a closed 
