This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A very useful device for testing out fixtures before they are connected up can be easily made as follows: Two wires are run from a plug, A, one to A socket, B, and the other to terminate at C. The line from the other side of the socket B terminates at D.
Ill: One Line of the Two Connecting Wires is Broken and the Ends Used as Terminals on the Fixture
In testing a fixture, the plug A Is turned into a socket of some source of current, and a lamp is turned into the socket B. The terminal C is held to the metal covering of the fixture, while the end D is held to one of the wires. If there is a leak of current, the lamp at B and those of the fixture will light up. - Contributed by Fred Schumacher, Brooklyn, N. Y.