It is extremely important to have no moving pieces of mag-
netic material about, and to avoid all draughts, which might affect
the magnetometer needle. For very accurate work other correc-
tions are needed, namely, for the magnitude of the arc of oscilla-
tion in diminishing the time T of vibration, for the torsion of
the suspensions, air churning and damping, and variations in tem-
perature altering the magnetic moment during the test. These
are, however, all very small quantities, and no account need be



Practical Electrical Testing. 23

taken of them, with fairly delicate means at hand in testing, in
order to obtain an approximate value for H. The value' of
this so obtained will be the value which holds in the room the
test is made in, and may be somewhat different from the values
obtained for the year from tables. Using the same magnet m the
two cases, we have by oscillations —

MH = i"--^
T-
and by deflections —

^- = --^ tan e

H 2d

using the '' A. position"

Hence, dividing, we get —

T-(r/- - /-)- tan e

and if D = the scale-deflection corresponding to an angular
motion (f of the needle, then —

_ /47r°K2//.2L _ 47r /K;/L

Note. — T = -, where n = the number of transits per second.
n f

Apparatus. — Delicate magnetometer with accessories (p. 337);
instrument for taking vibrations (p. 335) ; micrometer gauge
sensitive chemical weighing balance ; stop-watch ; three or four
similar permanently magnetized steel needles about 10 cms. long
and 1 cm. diameter.
