Electrical installation is the work of an expert, and the amateur is not advised to risk disaster by attempting it. It is well, however, to know something about the science which enables us to obtain light and heat thus conveniently.
The flow of an electric current is Nature’s method of restoring equilibrium when natural forces have been displaced. A dynamo does not create electricity and pump it along the wires. What it does is to create what is known as a difference of potential between two points which are called the poles or positive and negative terminals of the dynamo. On electric instruments the positive terminal is usually denoted by the sign + and the negative by —.
If we carry a weight up to the top 104 of a house, we expend energy in overcoming the force of gravity attracting the weight to the centre of the earth. In other words, we create a difference of potential. If we drop the weight out of the window, the force of gravity pulls it down to the ground. We cannot see gravity and we cannot see electricity, but we know what they can do. The dynamo creates between its poles, a condition corresponding to that which we create when we lift a weight, but electric current must have a conductor before it can flow. If we connect the two poles or terminals of a dynamo by a wire, electricity flows and restores equilibrium—that is, of course, supposing the dynamo is working.
If in the wire connecting the positive and negative terminals of a dynamo or of a battery, we interpose a motor or other electrical appliance, the current can be made to operate it.
Conductors and Insulators.
It is now clear that the conductor is that which permits a current to pass along it ; an insulator is that which prevents the flow of the current. There are neither perfect conductors nor insulators ; all offer some resistance, but all allow some slight flow across them, but the best insulators are those whose resistance to electric current is so great, that the leakage across them is inappreciable.
The best insulator is air, which, when one thinks of it, is fortunate; but china, glass, rubber, ebonite, &c., so long as they are dry, are excellent insulators.
Water is a tolerable conductor —although perfectly pure water, an almost impossible condition in the free state, is an insulator—so that the film of moisture over an insulator reduces its resistance, and therefore its efficiency. The best conductors are metals, and the one in greatest demand for supply mains is copper, while brass, its harder alloy, is used for movable parts of switches and the like.
How to Measure.
The Volt is the unit of measurement of the pressure of an electric current. The difference of potential, set up by a dynamo or by a battery, is not the same in every case, but may vary enormously, and the pressure of the resulting electric current varies accordingly, much as water pressure in a pipe varies, according to the height of the storage tank or reservoir from which the supply of water is derived. But this pressure must not be mistaken for the amount of the current.
The Ampere is the unit of measurement of the amount of current passing.
Resistance. Although an electric current requires a conductor to enable it to flow, even the best of conductors, such as copper, offer a certain amount of resistance to the current, just as, although a pipe is necessary to convey water from a reservoir to a house, there is friction between the water and the pipe which tends to restrict its flow. The unit of resistance is the Ohm. If we allow too much current to flow through a wire, the wire offers so much resistance that it become hot ; it may become red or white hot and fall to pieces.
Now the three terms—Volt, Ampbre and Resistance—although denoting different qualities, are interdependent. The amount of work that could be done by water flowing through a pipe depends upon the water pressure, the size of the hole in the pipe, and the frictional resistance of the pipe. Similarly, the power value of the current that flows along a conductor depends upon the pressure (voltage) , the amount (number of amperes), and the size and resistance of the conductor. Knowing these we can calculate the power value of the current, which is denoted in watts, 746 watts being equal to one horsepower and 746 watts of current for one hour being equal to one horse-power for one hour. A kilowatt is equal to one thousand watts.
If for some reason, the insulation on a wire becomes inefficient, and the current finds some easier path to earth than through the lamps and round its ordinary circuits, much more current passes along the wires than is intended. The mains rapidly heat and perhaps cause a fire. This is called a short circuit. The evil effects of a short circuit are prevented by a fine copper or tin wire fuse in the mains, which melts in the event of a short circuit, and disconnects the current.
Speed of.— -Electricity travels with the same speed as that of light and of electro-magnetic waves, namely about 186,000 miles per second—sixteen million times faster than an express train. FIRE-PROOFING.
Thin textiles can be rendered fire-proof by soaking them in a solution containing half a pound of sulphate of ammonia and one pound of sal-ammoniac dissolved in a gallon of water. After soaking, hang them out to dry.
Wood is rendered fire-proof by treating with silicate of soda.
The London County Council and other local authorities lay down stringent rules regarding the fire-proofing of all inflammable materials in public buildings, as theatres. Scenery and wardrobes are subject to periodical inspection and tests, and severe penalties are inflicted for any contraventions of the fire-proofing order.