from the influence of the Harth’s Magnetism. 491 
lower one.” (Electricity and Magnetism, Clerk Maxwell, vol. 
li, p. 105. 
It is desirable in many investigations on magnetism to free a 
needle not only from the inductive action of the earth, but also 
from its directive influence. The method that I have adopted 
Is the following: An ordinary tangent galvanometer is so con- 
structed that the coil is free to turn about a vertical axis, and also 
abouta horizontal one. The coil is first placed in a plane perpen- 
dicular to the magnetic meridian ; itis then turned about its hori- 
zontal axis until the component of the strength of the current act- 
Ing in the horizontal plane shall be just equal and opposite to the 
horizontal component of the earth's magnetism. In fig. 1, if 
we denote by 4 the angle which the plane of the coil makes 
with a vertical plane, and by a 
strength S of the current passing 
through the coil of the galvanometer ; 
we shall have S cos#=y, in which 
is the counter-balanced force of the 
earth’s magnetism. Under these con- 
ditions the magnetic needle is evi- 
dently in a state of unstable equili- 
brium, and, if placed perpendicularly 
p tothe magnetic meridian, will vibrate 
a 
to the attraction in the field of force in which it may be plac 
A bar of soft iron placed in the vertical plane which passes 
through the suspension of the needle perpendicular to the mag- 
netic meridian, at a suitable distance from one of the poles 
of the needle, serves to bring it back to zero, just as the con- 
troling magnet serves a similar purpose in Thomson's reflecting 
galvanometer, This method constitutes practically a new as- 
tatic system, 
It is often desirable to test the changes which take place in 
a closed circuit in which the full force of a current acts. Such 
as the effect of heating the electrodes (Faraday’s Experimental 
Researches in Electricity, vol. i, s 1637), the effects of induc- 
tion, and similar qualitative experiments In the use of a 
