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._
Arc lighting, 2/5 gas; with interest, ⅔.
Incandescent, ⅔ gas; with interest, 4/3.
_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._
Millimetre = 0·039 inches 1 mill = ·0254 millimetres.
Centimetre = 0·393 ” 1 inch = 2·5399 centimetres.
Decimetre = 3·93 ” 1 foot = 3·3480 decimetres.
Metre = 39·37 ” 1 yard = ·91439 metres.
Cubic metre = 35·32 cubic feet or 1·31 cubic yards.