of the Movable-coil Galvanometer. 83 
at any place, as we have seen; but it would undoubtedly be 
very convenient to have the galvanometer of equal sensibility 
everywhere. For that purpose an auxiliary magnet is placed 
east or west from the needle in a plane parallel to the meri- 
dian, which can turn round a horizontal axis passing through 
its neutral point and the centre of the needle, and being at 
right angles to the diameter on which the coil is turned. 
This magnet does not affect the zero position, and moreover, 
if placed exactly vertical with its magnetic axis, it does not 
alter the original constant, which then only depends upon the 
horizontal terrestrial component, more or less modified by the 
surroundings; but if it is dipped, the horizontal force acting 
on the needle is either augmented or diminished, according to 
the direction in which the magnet is turned and to the amount 
of dip given. It is easily seen that the magnetic influence of 
the surroundings upon the needle may now greatly vary from 
one place to another and still be compensated by the magnet, 
thus keeping the so-called " constant " of the galvanometer 
actually at a constant value, adjusting it, for instance, always 
so that the unit deflection of 45° with the vertical ring 
corresponds to a round number of volts and amperes, say 
five or ten. Under such conditions the deflection-scale could 
at once give the current or E.M.F. in amperes or volts. A 
gradual change of magnetism in the compensating magnet 
does not affect the measurements. I shall not further enter 
into this subject here, as I intend discussing it more fully on 
a future occasion. 
Methods of Measukement. 
For measuring current-strength or electromotive force 
either of the following four methods can be employed accord- 
ing to circumstances, viz.: — 
I. The General Method. — Applicable under almost any 
conditions. The coil is placed in such a position that the 
deflection attains a proper value. If a is the deflection of the 
needle, and <6 that of the coil from the meridian, the formula is 
x = tan a x sec <j> x const. ; 
or in case the multiplying powers P on the quadrant are used, 
the formula becomes 
a?=tan « x P x const. 
II. Method of Equality. — With this method the coil is each 
time placed in such a position that the needle is deflected 
exactly to the same angle yjr to which the coil is inclined, 
giving the formula 
x = tan -ty x sec ty x const. 
