ps of wood 1/4-in. thick 5/8-in. wide and 11 in.
long across the sides of the ring with their upper edges passing
exactly through the center of the ring. An ordinary pocket
compass, about 1-1/4 in. in diameter, is fitted in these strips so
that the center of the needle or pointer will be exactly in the
center of the ring and its zero point mark at the half-way point
between the two strips. Put the ring in place on the base, as
shown in the sketch, and connect the two ends of the wire to two
binding-posts that are previously attached to the base. Coat the
entire surface with brown shellac. Any deviation from the
dimensions will cause errors in the results obtained by its use.

Remove all pieces of iron or steel and especially magnets in the
near vicinity of the instrument when in use. Place the
galvanometer on a level table and turn it until the needle,
pointing north and south, and swinging freely, lies exactly in the
plane of the coil, as shown in the cut. The needle then will point
to zero if the directions have been followed closely. Connect one

[Illustration: Tangent Galvanometer]

cell of battery to the instrument and allow the current to flow
through the coils. The needle of the compass will be deflected to
one side or the other, and will finally come to rest at a certain
angle-let us say 45 deg. The dimensions of the instrument are such
that when the deflection is 45 deg. the current flowing through
the coils upon the ring is 1/2 ampere. The ampere is the unit
chosen to designate the strength of the electric current. For
other angles the value of the current may be found from the
following table:

Angles Degrees Current Amperes
10 .088
20 .182
30 .289
40 .420
45 .500
50 .600
55 .715
60 .865
70 1.375

As the magnetic force that acts upon a magnet needle varies in
different places the values given for the current will not be true
in all parts of the country. The table gives correct values for
the immediate vicinity of Chicago and that part of the United
States lying east of Chicago, and north of the Ohio river. The
results given should be multiplied by 1.3 for places south of the
Ohio river and east of the Mississippi.

** Home-Made X-Ray Instrument [151]

Two cylinders, AA, are mounted on a base, B, and mirrors, CC, are
fitted at an angle of 4