e Air Thermometer [152]

The illustration shows the complete thermometer. The water in the
glass tube is caused to rise and fall by the expansion and
contraction of the air in the tin box. A paper-fastener box, about
1-1/4 in.

[Illustration: Air Thermometer]

deep and 2 in. in diameter will serve very well for the box A.
Solder in the side of the box 1-in. piece of 1/4-in. brass tubing,
B, and then solder on the cover, C, so that the only escape for
the air is through the brass tube. Secure a piece of 1/4-in. glass
tubing - not shorter than 18 in.--and bend it as shown at D in the
sketch. Hold the part of the tube to be bent in the broad side of
a gas jet, and in a minute or two the tube will bend with its own
weight. Any angle can be given glass tubing in this way. Connect
the glass tube to B with a short piece of rubber hose, E. If the
hose is not a tight fit, bind with a short piece of fine copper
wire. The standard, F, is made from a piece of No. 10 wire about
10 in. long. To this standard solder the supporting wire, G--No.
14 wire will do. On one side bend the wire around the tube B, and
on the other around the glass tube, D.

The base, H, can be made of oak, stained and varnished. The bottom
of the box, A, is covered with lampblack so as to readily absorb
all heat that strikes the surface. The black should not be put on
until just before you paint the supports, cover and rim of the box
with gold or silver paint. Hold the bottom of the box to be
blackened over a little burning cotton saturated with turpentine.

The scale on the glass can be etched with hydrofluoric acid, or
made with a little black paint. The water can be put in with a
medicine dropper. This instrument will measure the amount of heat
given by a candle some 20 or 30 ft. away.
--Contributed by J. Thos. Rhamstine.

** Home-Made Battery Voltmeter [153]

Secure a piece of brass tube 3 in. long that has about 1/4-in.
hole. Put ends, A, 1-1/4 in. square and cut from heavy cardboard
on this tube. Make a hole in the center of each cardboard just
large enough to allow the brass tube to fit tight. Put on two or
three layers of stout paper around the brass tube and between the
cardboard ends. Wind evenly about 2 oz. of No. 26 cotton covered
magnet wire on the paper between the ends and leave about 2 in. of
wire on each end extending from the coil. Use a board 1/2- in.
thick, 3 in. wide and 6 in. long for the base and fasten the coil
to it, as s