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166 Forging- Stamping - Heat Treating
H E A T T R E A T M E N T and M E T A L L O G R A P H Y of STEEL
A P r a c t i c a l C o u r s e in t h e E l e m e n t s o f
PART 4.
GALVANOMETERS AND MILLIVOLTMETERS
Galvanometer.
W H E X an electric current passes through a wire,
lines of magnetic force are set up around the
wire. The lines take the form of circles, concentric
with the conductor, as illustrated in Fig. 90-a. It
does not matter whether the wire is bare or insulated.
A compass needle placed near a conductor carrying
a direct current will tend to take a position at right
angles to the conductor. If the direction of the current
is reversed, the compass will reverse its position,
end for end, showing that the lines of force have
direction. If the conductor is formed into a loop or
coil, as illustrated in Fig. 90-b, the lines of force will
pass through the space enclosed by the coil, all in one
direction. The coil will then act like a magnet having
a north pole at one side and a south pole at the other.
(Such a magnet is known as a "solenoid." These rules
apply to direct current, not alternating current.)
Increasing the number of turns in the coil will increase
the magnetic strength for a given current. If
the coil is allowed freedom to move, by suspending it
between long fine filaments, which also serve as conductors,
as shown in Fig. 90-c, it will behave like a
compass. One side of the coil will tend to face the
north and is called the "north pole", the opposite side
is called the "south pole'.. The lines of magnetic
force are assumed to enter at the south pole and leave
*The author wishes to acknowledge his indebtedness to the
following references for material contained in this chapter, and
to recommend them to the student for further reading: (9)
"Measurement of High Temperatures," Burgess & LeChatelier;
(10) "Pyrometric Practice," Foote, Fairchild & Harrison, Technologic
Paper of the Bureau of Standards, No. 170. (Obtainable
from Supt. Documents, Govt. Printing Office, Washington, D. C
60 cents.) (11) Pyrometry Data Sheets, A. S. S. T.
The author is Chief Metallurgist, Naval Aircraft Factory,
United States Navy Yard, Philadelphia, Pa.
Copyright, 1925, by H. C. Knerr.
P h y s i c a l M e t a l l u r g y
May, 1925
at the mirth pole, as shown by the arrows in the top
view. If the coil is brought near to a permanent magnet,
its north pole will be attracted by the south pole
of the magnet, and its south pole will be repelled by
the south pole of the magnet. Opposite poles attract
and like poles repel each other.
The magnetic field of the earth is comparatively
weak. If the coil is suspended between the poles of a
strong permanent magnet, as shown in Fig. 91-a, the
turning force will be greatly increased. The heavy
arrow represents the magnetic poles of the movable
coil. This is the principle of the d'Arsonval galvanometer,
and by such an instrument very small electric
currents can be detected. The coil is given a large
number of turns of fine insulated wire, and is suspended
by a thin filament of a springy metal, such as
phosphor bronze. A similar filament is attached to the
bottom of the coil. These "suspension wires", upper
and lower, serve to carry the current to and from the
coil. They also act as delicate springs, and when no
current is flowing, hold the coil in the neutral or zero
position, shown in 91-a. When a current passes
through the coil, it would turn to the position shown
at 91-b, were it not for the restraining action of the
spring suspensions. It therefore takes some intermediate
position between 91-a and 91-b, as at 91c. The
amount which the coil turns may be measured by attaching
a light pointer to it, and measuring the deflection
on a graduated scale.
The deflection of the coil will depend upon the following
factors:
1. The amount of current flowing through the
coil.
2. The number of turns in the coil.
3. The strength of the magnetic field in which
the coil is suspended.
4. The strength of the restraining springs or
suspensions.
If factors 2, 3, and 4 are kept constant, the deflection
will depend solely upon the amount of current