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Before switchboards were used.
used, except for "arc" lighting. As the continuous current systems are the most simple we will start the discussion with these, taking up first the constant potential switchboard.
The switchboard, in the strictest sense of the word, is not an indispensable adjunct of an electrical installation, and in fact was unknown for several years after the introduction of electric lighting. It first came into use, in a very crude form, as a matter of convenience; it was gradually elaborated, and at the present time is regarded as a part of the system only second in importance to the generator itself. It belongs to that vast class of apparatus that first come into use as a luxury and finally come to be regarded as a necessity. At the present time we would consider it impossible to operate a boiler without a steam gauge, but they were so used in days gone by, and if we had no gauge we could get along without it. These remarks are equally applicable to the electrical switchboard.
In the early days of “arc” lighting the wires from the binding posts of the dynamo were run directly out to the lamp circuit, in most cases not even a switch being used. Whenever it became necessary to discontinue the lights, the dynamo was stopped. Before long it became evident that a switch introduced into the circuit would be a convenience, as then it would be possible to put out the lights without stopping the dynamo, and this certainly would be advantageous in cases where it was desired to suspend the lights for only a few minutes. In this way the switch for opening the line came into use. Very early in the history of “arc” lighting it was discovered that unless the current remained practically uniform, the Voltmeter and ammeter.
lamps would not burn steadily; hence it became desirable to have an instrument whereby the current strength could be indicated, and this brought into use the ammeter. With incandescent lighting it was soon found that uniformity of pressure was the point of vital importance, and thus the voltmeter was introduced in such installations. In “arc” lighting the current remains of constant strength, therefore there is no danger of burning out the generator by overloading. In the incandescent lighting system the case is different; each lamp adds its quota to the current strength required, and if the number of lamps is sufficiently increased, the current will become strong enough to burn out the generator, providing there is enough power behind it to keep it running. To prevent burning out generators of the incandescent type, by overloading, safety devices were gotten up; these acting by opening the circuit and thus stopping the flow of current whenever the danger limit is reached. There are two types of safety devices used to avoid the destructive effects of overloads; one is the “safety fuse,” which is simply a piece of wire of such size that it will be melted by a current of sufficient strength to injure the generator, and the other is the “circuit breaker,” which is actuated. by a magnet, the strength of which is exerted in opposition to a spring or some other counteracting force. The strength of the magnet increases with the current, and when the predetermined point is reached, the device acts, as at that point the force of the magnet is sufficient to overcome the reactive force.
Generators used for incandescent lighting are designed to develop the required pressure at a certain velocity of rotation, but it frequently happens that for one reason
Instruments needed in a circuit.
or another it is not practicable to obtain just this speed. In order that the generator may be able to develop the desired voltage, even if the velocity is somewhat varied, a device is provided by means of which the pressure may be increased or decreased without changing the velocity. This device is simply a resistance, or rheostat, and is called a field regulator. If the voltage of the generator is too high, a sufficient portion of the regulator resistance is thrown into the circuit of the generator field coils to bring it down to the required point, and if the voltage is too low the regulator resistance is cut out.
From the foregoing it can be seen that to successfully operate a single constant potential generator all we require is a field regulator, a circuit breaker, an ammeter, a voltmeter and a switch to open the circuit. To this we should also add switches for opening independently the several branch circuits. From Fig. I an idea of the devices and the manner in which they are connected can be obtained.
Before proceeding to explain this diagram it will be well to show the connections of the generator independently of the distributing circuits, showing the connections for compound as well as shunt wound machines. In Fig. 2 the generator connections are shown for a simple shunt two-pole generator. At V a voltmeter is placed which indicates the e m f developed, and, as will be noticed, it is connected directly across the main lines by wires g g. At Am, an ammeter is connected in series in the line N, and it measures the strength of the current in amperes. At R, the field regulating rheostat is located, being placed as shown by wires f f in series in the shunt field coil cireuit.