** To Drive Away Dogs [106]

The dogs in my neighborhood used to come around picking up scraps.  After I connected up my induction coil, as shown in the sketch, we were not bothered with them.  A indicates the ground; B, switch; and C, a bait of meat, or a tempting bone.

[Illustration:  Shocking-Machine]

—­Contributed by Geo. W. Fry, 903 Vine St., San Jose, Cal.

** An Automatic Lock [106]

The illustration shows an automatic lock operated by electricity, one cell being sufficient.  When the circuit is broken a weight, A, attached to the end of the armature B, tends to push the other end of the armature into the screw eye or hook C, which is in the door, thus locking the door.

To unlock the door, merely push the button E, The magnet then draws the armature out of the screw eye and the door is unlocked.  The dotted line at D shows the position of the armature when the circuit is complete and the door unlocked.  The weight must be in proportion to the strength of the magnet.  If it is not, the door will not

[Illustration:  Automatic Electric Lock for Doors]

lock, or would remain locked.  The button can be hidden, as it is the key to the lock.  —­Contributed by Claude B. Melchior, Hutchinson, Minn.

** Experiment with Two-Foot Rule and Hammer [106]

An example of unstable equilibrium is shown in the accompanying sketch.  All that is needed is a 2-foot rule, a hammer, a piece of string, and a table or bench.  The experiment works best

[Illustration:  An Experiment in Equilibrium]

with a hammer having a light handle and a very heavy head.

Tie the ends of the string together, forming a loop, and pass this around the hammer handle and rule.  Then place the apparatus on the edge of the table, where it will remain suspended as shown.  —­Contributed by Geo. P. Schmidt, Culebra, Porto Rico, W. I.

** Simple Current Reverser [107]

On a block of hardwood draw a square (Fig. 1) and drill a hole in each corner of the square.  Fill these holes with mercury and connect them to four binding posts (Fig. 1).

On another block of wood fasten two wires, as shown in Fig. 2, so that their ends can be placed in the holes in the first block.  Then connect up with the

[Illustration:  Details of Reverser]

motor and battery as in Fig. 3.  When the block is placed on with the big arrow A pointing in the direction indicated in Fig. 3, the current flows with the small arrows.  To reverse turn through an angle of 90 degrees (Fig. 4). —­ Contributed by F. Crawford Curry, Brockville, Ontario, Canada.

** Alarm Clock to Pull up Furnace Draft [107]

A stout cord, A, is attached to the draft B of the furnace, run through a pulley, C, in the ceiling and has a window weight, D, attached at the other end.  A small stick is put through a loop in the cord at about the level of the table top on which the alarm clock F stands.  The other end of stick E is placed under the key G of the alarm clock.  When the alarm rings in the early morning, the key turns, the stick