the same way as if we were to say of water, that a current of 10 gallons per minute was flowing through a pipe.

Electro-Motive Force-Pressure or Potential.—These terms are all used in the code to express the same thing. We will use the more common term of "electro-motive force," which is commonly abbreviated to E. M. F. E. M. F. may be defined as a force which causes, or tends to cause, a current of electricity to flow. To use our same analogy, suppose we have a tank full of water; we shall have upon the bottom of a tank a pressure due to the head. If we bore a hole into the bottom of the tank, we shall immediately have a flow of water. If we place our hand over the hole, the pressure will still tend to cause a flow and the flow will be instantaneous as soon as the obstacle is removed. In electricity we measure the pressure by an arbitrary unit called a "volt." This corresponds with the pounds. of pressure to the square inch with water. With water, the greater the pressure the greater will be the flow, the outlet remaining the same; so with electricity. The greater the E. M. F., the greater will be the current, provided other conditions remain unchanged. In fact, with electricity, the relation of flow to pressure is more simple than with water, for, other things remaining the same, if we double our pressure, we double our current, i. e., the current will be exactly proportional to the pressure; or the amperes will be proportional to the volts.

Dynamo. A dynamo is a machine which, when driven by an engine or other source of power, trans

forms the work of the engine or prime mover into electricity. It may be compared to a pump driven by a belt. When motion is transmitted to the pump, it sets up a pressure which will cause, or tend to cause, a flow of water. So, when a dynamo is set in motion by any mechanical means, an electrical pressure will be set up, and this is called the " pressure or "E. M. F." of the machine. This pressure will tend to cause a flow of electricity, or an electrical current.

A dynamo is a reversible machine, i. e., it can be used to generate a current of electricity, or, if the current of electricity is produced by another source, and sent through it, its armature or moving part will be set in rotation and it can be used as a source of power to drive other machines. When thus used it is called a "motor." The distinction between a dynamo and a motor is one of application, and not necessarily of construction. To make the distinction clear, it is now common to speak of a machine which generates electricity as a "generator," and of one which transforms electricity into mechanical work as a "motor." The words "dynamo" and "generator" are used in the code to mean the same thing.

Conductor. Unlike water, electricity does not flow most readily when not impeded by a solid substance. In fact, while no ordinary pressure will cause electricity to pass through the air, a small pressure may cause an immense current to flow through a mass of metal. therefore say that metal is a conductor of electricity, and that air is a non-conductor.

We

When we wish to direct a current of water from one

point to another, we provide a path free from solid. obstruction and we confine the water through this path by some solid substance, such as the bank of a canal or a metal pipe. With electricity, however, we provide a metallic path, such as a copper wire, and the electricity is kept from leaving the wire by the surrounding air, or some other non-conducting material which separates the wire or metal path from other paths into which it might flow if there was no such barrier.

Resistance. Although electricity will readily flow in a metal wire, a given pressure will not produce the same flow in wires of different metals, or in different sized wires of the same metal. Just as with water, a given pressure will not send the same amount through a small hole as through a large one, or through a long pipe of small diameter, as through a short one of large diameter, so a given E. M. F. will send a small current through a long, thin wire, and a strong current through a short and thick one. We explain the different results by saying that the long, thin wire offers a "resistance" to the flow of the current. This we call "electrical resistance," or simply "resistance." Resistance is measured in “ohms,” an ohm being an arbitrary unit.

Electrical resistance may be compared to friction in a pipe carrying a current of water; the greater the pressure, and the less the friction, the greater will be the flow. So, in electricity, the greater the E. M. F., and the less the resistance in the path or conductor, the greater will be the current. Or, to express the same thing in electrical terms, the greater the voltage of our Ivnamo, and the less the resistance of our conductor

in ohms, the greater will be the current in amperes which will flow through the conductor.

A

Insulator.-If we are to confine electricity to cur "conductor," we must separate it from other conductors; or, as we say, we must "insulate" it. This is accomplished by surrounding our conductor with a non-conducting substance, or by supporting it by a non-conducting substance in the air, which is itself a non-conductor. Any such non-conducting material used to support or surround a wire is called an insulator. In practice it is customary to speak of a nonconducting support as an "insulator," and of a nonconducting material surrounding a conductor or wire as "insulation." It is evident that the distinction between "insulators" and "conductors" is simply relative. non-conducting or insulating substance is simply a substance of very high resistance, but the resistance of materials used for insulation are so enormous, that we are justified in calling them "non-conductors." Of all insulators, dry air is the best, and dry glass the next best; rubber, porcelain, oil, shellac, mica, paper, cotton, silk, etc., are the substances most commonly used. As safety in electrical work depends solely upon the confining of the current to its proper circuit, the problem of safety is very largely one of insulation, and it will be seen that the greater part of the "code" is devoted to specifying material and methods which will secure good insulation.

Polarity.

The flow of current from a dynamo is not exactly analogous to the flow of water from a tank, since to have a continuous current of electricity, we must

have a continuous conducting circuit; i. e., the current must flow from the dynamo through the circuit and back again to the dynamo. In this respect the dynamo is more like our pump. We must continually supply water to the pump in order to have a continuous flow. Any electrical circuit must be continuous in order to have a current, and we may, if we like, imagine the condition similar to that of water flowing round and round in an endless pipe. We assume that any ordinary current flows always in the same direction, and that the current from a dynamo goes out from the machine on one conductor, and back to the machine on another. We express this by saying that these two conductors are of "opposite polarity," and the points where they join the dynamo we designate as the two "poles" of the machine. The pole where the current emerges is called the "positive" pole, and the one to which it returns is called the "negative" pole. "Polarity" is naturally relative, and we speak of any part of a circuit as being positive with reference to another, when the current flows from the first point to the second, or when the pressure tends to set up such a current. Other electrical terms which appear in the "code" will be defined as we come to them.