4, it should be filed a
little at one end until it assumes the position indicated. The
pointer or hand, Fig. 5, is made of wire, aluminum being
preferable for this purpose, although copper or steel will do.
Make the wire 4-1/2 in. long and make a loop, D, 1/2 in. from the
lower end. Solder to the short end a piece of brass, E, of such
weight that it will exactly balance the weight of the hand. This
is slipped on the pivot, and the whole thing is again placed in
position in the support. If the pointer is correctly balanced it
should take the position shown in Fig. 1, but if it is not exactly
right a little filing will bring it near enough so that it may be
corrected by the adjusting-screw.

Next make a brass frame as shown in Fig. 6. This may be made of
wood, although brass is better, as the eddy currents set up in a
conductor surrounding a magnet tend to stop oscillation of the
magnet. (The core is magnetized when a current flows through the
instrument.) The brass frame is wound with magnet wire, the size
depending on the number of amperes to be measured. Mine is wound
with two layers of No. 14 wire, 10 turns to each layer, and is
about right for ordinary experimental purposes. The ends of the
wire are fastened to the binding posts B and C, Fig. 1.

A wooden box, D, is then made and provided with a glass front. A
piece of paper is pasted on a piece of wood, which is then
fastened in the box in such a position that the hand or pointer
will lie close to the paper scale. The box is 5-1/2 in. high, 4
in. wide and 1-3/4 in. deep, inside measurements. After everything
is assembled put a drop of solder on the loop at D, Fig. 5, to
prevent it turning on the axle.

To calibrate the instrument connect as shown in Fig. 7, where A is
the homemade ammeter; B, a standard ammeter; C, a variable
resistance, and D, a battery, consisting of three or more cells
connected in multiple. Throw in enough resistance to make the
standard instrument read 1 ohm [sic: ampere] and then put a mark
on the paper scale of the instrument to be calibrated. Continue in
this way with 2 amperes, 3 amperes, 4 amperes, etc., until the
scale is full. To make a voltmeter out of this instrument, wind
with plenty of No. 36 magnet wire instead of No. 14, or if it is
desired to make an instrument for measuring both volts and
amperes, use both windings and connect to two pairs of binding
posts.
--Contributed by J.E. Dussault, Montreal.

** How to Make a T