In operation, the device must stand on end and should be connected in the circuit as shown in the sketch.  When the electrical waves strike the needle, the conductivity of the filings is established and a click is heard in the receiver.  —­Contributed by Carl Formhals, Garfield, Ill.

** One-Wire Telegraph Line [78]

The accompanying wiring diagram shows a telegraph system that requires no switches and may be operated with open-circuit batteries on a one-wire

[Illustration:  Diagram of One-Wire Line]

line with ground connections at each end.  Any telegraph set in which the key makes double contact can be connected up in this way.  —­Contributed by R. A. Brown, Fairport, N. Y.

** How to Make a Water Rheostat [78]

A water rheostat may be made by fitting a brass tube with a cork, through which a piece of wire is passed.  The brass tube may be an old bicycle hand pump, A (see sketch), filled with water.  Pushing the wire, B, down into the water increases the surface in contact, and thus decreases the resistance.  An apparatus of this kind is suitable for regulating the current from an induction coil, when the coil is not provided with a regulator, and by using a piece of pipe instead of the tube, it can be used to regulate the speed of a motor.

When the pipe is used, a piece of brass or copper rod should be substituted for the wire, in order to increase the surface.  Adding salt to the water will decrease the resistance, and, when used with a motor, will give a greater speed.  —­Contributed by John Koehler, Ridgewood, N. J.

[Illustration:  Diagram of One-Wire Line Water Rheostat]

** Electric Door-Opener [78]

A very convenient and efficient device for unlocking any door fitted with a spring lock is shown in the accompanying sketches.  A fairly stiff spring, A, is connected by a flexible wire cord to the knob B. The cord is also fastened to a lever, C, which is pivoted at D and is released by a magnetic trigger, E, made from the armature and magnet of an old electric bell.

When the circuit is completed by means of a secret contact device outside the door, the magnet, F, pulls down the armature, which releases the trigger and allows the spring to open the lock.  If there are metal numbers on the outside of the door they may be used for the secret contact, if desired,

[Illustration:  Apparatus Placed on Inside of Door]

but if there are no numbers on the door, a small contact-board may be constructed by driving about 12 brass headed tacks into a thin piece of wood and making connections at the back as shown in the wiring diagram.

In this particular diagram the tacks numbered 1 and 7 are used for unlocking the door, the others being connected with the electric-bell circuit as indicated, for the purpose of giving an alarm should anybody try to experiment with the secret contacts.  By means of a pocket knife or other metal article the operator can let himself in at any time by connecting the tacks numbered 1 and 7, while a person not knowing the combination would be liable to sound the alarm.  Of course, the builder of this device may choose a combination of his own and may thus prevent anybody else from entering the door, even those who read this description.  —­Contributed by Perry A. Borden, Gachville, N. B.