To calibrate the instrument connect as shown in Fig. 7, where A is the homemade ammeter; B, a standard ammeter; C, a variable resistance, and D, a battery, consisting of three or more cells connected in multiple.  Throw in enough resistance to make the standard instrument read 1 ohm [sic:  ampere] and then put a mark on the paper scale of the instrument to be calibrated.  Continue in this way with 2 amperes, 3 amperes, 4 amperes, etc., until the scale is full.  To make a voltmeter out of this instrument, wind with plenty of No. 36 magnet wire instead of No. 14, or if it is desired to make an instrument for measuring both volts and amperes, use both windings and connect to two pairs of binding posts.  —­Contributed by J.E.  Dussault, Montreal.

** How to Make a Three-Way Cock for Small Model-Work [50]

[Illustration:  Three-way Valve]

In making models of machines it is often necessary to contrive some method for a 3- or 4-way valve or cock.  To make one, secure a pet cock and drill and tap hole through, as shown in the cut.  If for 3-way, drill in only to the opening already through, but if for a 4-way, drill through the entire case and valve.  Be sure to have valve B turned so as to drill at right angles to the opening through it.  After drilling, remove the valve, take off the burr with a piece of emery paper and replace ready for work.

** Easy Experiments with Electric-Light Circuit [50]

An electric-light circuit will be found much less expensive than batteries for performing electrical experiments.  The sketch shows how a small arc light and motor may be connected to the light socket, A. The light is removed and a plug with wire connections is put in its place.  One wire runs to the switch, B, and the other connects with the water rheostat, which is used for reducing the current.

[Illustration:  Arc-Light Motor and Water Rheostat]

A tin can, C is filled nearly to the top with salt water, and a metal rod, D, is passed through a piece of wood fastened at the top of the can.  When the metal rod is lowered the current increases, and as it is withdrawn the current grows weaker.  In this way the desired amount of current can be obtained.

By connecting the motor, E, and the arc light, F, as shown, either one may be operated by turning switch B to the corresponding point.  The arc light is easily made by fastening two electric light carbons in a wooden frame like that shown.  To start the light, turn the current on strong and bring the points of the carbons together; then separate slightly by twisting the upper carbon and at the same time drawing it through the hole.

** How to Make an Interrupter [51]

The Wenult interrupter is an instrument much used on large coils and is far more efficient than the usual

[Illustration:  Details of Interrupter]

form of vibrators.  It can also be used with success on small coils as well as large.  Although it is a costly instrument to purchase, it can be made with practically no expense and the construction is very simple.