assembling.)

The wiring diagram, Fig. 3, shows how the connections are made.
This method of connecting is suitable for all coils up to 1-1/2
in. spark, but for larger coil better results will be obtained by
using an independent type of interrupter, in which a separate
magnet is used to interrupt the circuit. Besides the magnetic
vibrators there are several other types, such as the mercury
dash-pot and rotary-commutator types, but these will become better
known to the amateur as he proceeds in his work and becomes more
experienced in coil operation.

** Combined Door Bell and Electric Alarm [114]

This device consists of a battery and bell connection to an alarm
clock which also acts as a door bell, the whole being mounted on a
board 18 in. long and 12 in. wide. Referring to the sketch
accompanying this article, the letters indicate as follows: A,
bell; B, battery ; C, switch; D, V-shaped copper strip; E, copper
lever with 1-in. flange turned on one side, whole length, 4 in.;
F, spring to throw lever E down in V-shaped piece to make
connection; G, lever to hold out E when device is used as a door
bell; lines H, go, one from bell, A, and one from battery, B, to
the door; I, shelf for clock.

See that the ring in the alarm key of the clock works easily, so
that when it is square across the clock it will drop down. Fasten
a piece of copper about

[Illustration: Wiring Disgram]

1 in. long to key, then wind the alarm just enough so that the key
stands straight up and down. Place the clock on the shelf and the
key under the flange of lever E. Pull lever G down out of the way
and close the lever on the switch. The alarm key will turn and
drop down, letting lever E drop into the V-shaped piece D and make
connection.

For the door-bell connection close lever on switch C, and put G up
so that D and E do not come in contact. If anyone is ill and you
do not want the bell to ring, open switch C.

The wiring for this device may all be on the back of the board.
The switch and levers are fastened with small screw bolts, which
allows wiring at the back. Saw two spools in half and fasten the
halves to the four corners of the board at the back, and the
apparatus may be put up where one likes.

**v To Build a Small Brass Furnace [115]

Bend a piece of stout sheet iron 23 in. by 12 in. round so that
the inside

[Illustration: Furnace]

diameter is 7 in., and then rivet the seam. Fit in a round piece
of sheet iro