OTHER DETAILS.

The fly wheel may be a disc of wood.

Oil all the rubbing parts slightly.  Connect T1 to one terminal of the battery, T2 to the coil, and the other terminal of the battery to the coil.  Set the engine going.  If it refuses to run, make sure that B1 is pressing against D. The speed of the engine may possibly be improved by careful adjustment of B2 and an alteration in the setting of the crank, and will certainly be accelerated by increasing the number of battery cells.

The cost of the engine described was about 1s, 3d., exclusive of the battery.

XII.  AN ELECTRIC ALARM CLOCK.

Anybody who possesses an alarm clock with an external gong, an electric bell, and a battery, may easily make them combine to get the drowsiest of mortals out of bed on the chilliest of winter mornings.  The arrangement has as its secondary advantages and capabilities—­

(l) That the clock can be placed where its ticking will not disturb the person whom it has to arouse in due course (some of the cheaper clocks are very self-advertising);

(2) That one clock can be made to operate any number of bells in different parts of the house.

The main problem to be solved is, how to make the alarm mechanism of the clock complete an electric circuit when the alarm “goes off.”

If you examine an alarm clock of the type described, you will find that the gong hammer lies against the gong when at rest, and that its shaft when in motion vibrates to and fro about a quarter of an inch.

[Illustration:  Fig. 89.—­Plan of release gear of electric alarm, as attached to clock.]

Fig. 39 shows a. method of utilizing the movement of the hammer.  A piece of wood, 2 inches long, wide enough to fill the space between the rear edge of the clock and the hammer slot, and 1/2 inch thick, has its under side hollowed out to the curvature of the clock barrel.  This block serves as a base for two binding posts or terminals, T1 T2.  A vertical slit is made in T1 and in this is soldered [to] one end of a little piece of spring brass strip, 1 inch long and 1/4 inch wide.  To the back of the other end of the strip solder a piece of 1/20 inch wire, projecting l inch below the strip.  The strip must be bent so that it presses naturally against T2.  A little trigger, B, which you can cut out of sheet brass, is pivoted at a, where it must be raised off the base by a small washer.  It projects 1/4 inch beyond the base on the gong support side.  A square nick is cut in it at such a distance from a that, when the wire spike on C is in the nick, the strip is held clear of T2.  The other end of the trigger, when the trigger is set, must be 1/8 inch from the shank of the alarm hammer—­at any rate not so far away that the hammer, when it vibrates, cannot release C from the nick.