, U,
is fastened to D with brass screws, not with nails, as these would
affect the needle. It is placed at one side of D so that the compass
needle placed in the center of D will also be in the center of the wire
coils when used in App. 117. The table is fastened in any position by a
screw-eye, S I, which presses a copper washer, W, against U. S I works
through a narrow slot, S, and screws into the back of the galvanometer.
By making S longer, the table may be used for other laboratory purposes,
if it is joined with some other form of standard.

[Illustration: Fig. 124.]

APPARATUS 150.

_274. Glue Pot._ If you have occasion to use glue, you can make a good
glue pot out of 2 tin cans, one being placed inside the other. Put 1/4
teacupful of glue in the inside can. If you have time, cover it with
cold water, and let it soften. If you are in a hurry, cover it with hot
water. Set this inside can into the other, in which you have boiling
water. Do not let the water boil over. The solder will not melt from
ordinary tomato cans, if you keep water in them. Thin the glue with a
little hot water until it drips from the brush in drops. Have the glue
hot and fairly thin, and apply quickly. Hold the pieces of wood together
by pressure until the glue hardens.

APPARATUS 151.

_275. Paraffine Paper and Cardboard_ are extremely useful for insulating
purposes. The paraffine used in candles will do, if you cannot get it in
block form. While ordinary paper will do for simple apparatus to wind
about coils, etc., you will find that paraffine paper can be handled
very rapidly. To melt the paraffine you should use a double boiler, or
one made of a shallow basin set in a pan of water. The water should be
boiled. This will melt the paraffine in the basin. Strips of paper just
passed through the melted paraffine will become soaked, and the
paraffine will quickly harden in the air. Allow thick cardboard to soak
for a minute or two, to drive out all the air. This makes excellent
washers for electro-magnets. (See Sec. 119.) To make one piece of this
paper stick to another, merely pass a clean hot nail over the two where
they lap. To hold coils of wire together, or to wooden bases, use a few
drops of paraffine applied with a large hot nail.

276. Caution. Do not heat paraffine directly upon the fire or over a
burner, unless you watch it constantly. It will burn if its temperature
is raised too much. It is better to heat it with steam, as