rcelain
insulator of the carbon cylinder and also through the wire
connector. Pull the zinc up as far as it will go and tighten the
lower thumb screw so that it holds the wire secure. Place the
carbon in the jar. If the solution touches the zinc, some of it
should be poured out. To determine whether or not the zinc is
touched by the solution, take out the carbon and lower the zinc.
If it is wet, there is too much liquid in the jar. The battery is
now ready for use.

To cause a flow of electricity, lower the zinc until it almost
touches the bottom of the jar and connect an electric bell or
other electrical apparatus by means of wires to the two binding
posts.

This battery when first set up gives a current of about two volts.
It is useful for running induction coils, or small electric
motors. When through using the battery, raise the zinc and tighten
the lower thumb screw. This prevents the zinc wasting away when no
current is being used.
--Contributed by H. C. Meyer, Philadelphia.

[Illustration: Fig.1 Details of Homemade Battery]

** Door-Opener for Furnace [83]

The accompanying diagram shows an arrangement to open the coal
door of a furnace. When approaching the furnace with a shovelful
of coal it is usually necessary to rest the shovel on the top of
the ash door, while the coal door is being opened. With my device
it is only necessary to press the foot pedal, which opens the
door. After putting in the coal, pressing the pedal closes the
door. The pulley in the ceiling must be placed a little in front
of the door, in order to throw the door open after lifting it from
the catch. A large gate hinge is used to hold the pedal to the
floor.
--Contributed by Edward Whitney, Madison, Wis.

[Illustration: Furnace Door Opener]

** How to Make an Efficient Wireless Telegraph [84]
By GEORGE W. RICHARDSON

A simple but very efficient wireless telegraph may be constructed
at slight cost from the following description:

The sending apparatus consists of nothing but an induction coil
with a telegraph key inserted in the primary circuit, i. e., the
battery circuit. This apparatus may be purchased from any
electrical-supply house. The price of the coil depends upon its
size, and upon the size depends the distance signals can be
transmitted. If, however, one wishes to construct his own coil he
can make and use, with slight changes, the jump-spark coil
described elsewhere in this book. This coil, being a 1-in. c