To ascertain in what direction the electric current is flowing through any wire by means of a pocket compass:—

A current flowing from south to north will always deflect the needle to the west, providing the wire in which the current flows is over the instrument.

The word S. N. O. W. expresses this—south north over west; and should be remembered.

Another simple plan is to hold the outstretched right hand over the compass; then, if the current flows in the direction of the wrist to the fingers, the needle will move towards the thumb.

To find the direction of the current in the wire of an electro-magnet:

Place the palm of the hand on the coil with the fingers parallel to the wires: the thumb will point to the North Pole if the current is flowing as in previous rule towards the fingers. Conversely: if the North Pole is known, the fingers will point to the direction of the current when placed parallel with the wires, with the thumb pointing to the North Pole.

If no compass is available, take two pieces of lead and place a few inches apart in a pot containing dilute sulphuric acid, scrape the lead clean, and join a piece of wire to each and connect to poles to be tested. After current has passed a short time one piece of lead will become brown, the other grey; trace the former to the dynamo cable, and this is the positive, and should be marked with a + or be painted red for future distinction.

Incandescent Lights or Glow Lights.

The number of lights required to illuminate any room would vary very much, according to the style of decoration and position of the lamps. As a rule, a similar number of glow lamps are required as there would be gas burners. The former give a much higher standard of illumination, which, curiously enough, is generally expected with electric lighting on account of the purity of the atmosphere when the full light is being used, which is not the case with gas.

One 16 candle-power lamp will light an area of about 8 feet in diameter at 8 feet above ground; in ordinary situations allow, one lamp for 38 square feet.

Arc Lighting of Works.

External.—56. 2,000 CP. arc lights will illuminate 160,000 square yards, or one for each 2,800 square yards.

Internal.—43. 2,000 CP. arc lights will illuminate 31,500 square yards, or one for each 730 square yards.

Approximate Cost of Electric Light, Museum.

Motive Power.

Compound engine 2 lbs. of coal per indicated horse-power per hour.

Good single-acting engine 3 to 6 per indicated horse-power per hour.

An indicated horse-power can be obtained in a compound engine from 20 lbs. of steam per hour.

A good boiler evaporates 9 to 10 lbs. of water per lb. of coal.

From 21 to 28 cubic feet of gas are required in a gas-engine per indicated horse-power per hour.

Incandescent Lamps.

In practice allow 9-60 watt 16 CP. lamps per indicated horse-power of engine.

Mem. for Wire Running.

Leads to the left or “low,” “light coloured.”

Returns to the right or “raised,” “red.”

English and French Measures.