Useful Notes.
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.