may at first glance seem severe or trivial, but ery rules are the ones that have been originated ent the continuation of practices which have continual hazards and repeated losses.

CHAPTER XVI

CLASS D, ALTERNATING SYSTEMS. CONVERTERS OR

TRANSFORMERS.

TEXT OF THE Code CoveRED BY THIS CHAPTER. CLASS D, ALTERNATING SYSTEMS. CONVERTERS OR TRANSFORMERS. 33. CONVERTERS:-a. Must not be placed inside of any building, except the Central Station, unless by special permission of the underwriters having jurisdiction. b. Must not be placed in any but metallic or other non-combustible cases. c. Must not be attached to the outside walls of buildings, unless separated therefrom by substantial insulating supports.

34. In those cases where it may not be possible to exclude the converters and primary wires entirely from the building, the following precautions must be strictly observed: Converters must be located at a point as near as possible to that at which the primary wires enter the building, and must be placed in a room or vault constructed of, or lined with, fire-resisting material, and used only for the purpose. They must be effectually insulated from the ground, and the room in which they are placed be practically air-tight, except that it shall be thoroughly ventilated to the out-door air, if possible through a chimney or flue.

35. PRIMARY CONDUCTORS:-a. Must each be heavily insulated with a coating of moisture-proof material from the point of entrance to the transformer, and, in addition, must be so covered and protected that mechanical injury to them, or contact with them, shall be practically impossible. b. Must each be furnished, if within a building, with a switch and a fusible cut-out where the wires enter the building, or where they leave the main line, on the pole or in the conduit. These switches should be inclosed in

and fire-proof boxes perfectly outside the building kept apart at least ten inches, and at the same distance other conducting bodies when inside a building.

ECONDARY CONDUCTORS: -Must be installed according to for "Low Potential Systems."

far electric currents have been described only g analogous to currents of water flowing in a We have however in electricity to deal with two f currents, "direct" currents and "alternating" 5. A direct current may be compared to a curwater flowing constantly in one direction in a An alternating current is one which flows first in ection and then in the opposite direction. We m a conception of this action by imagining that a cylinder like a steam engine cylinder with a it. Suppose now that we bore a hole in one the cylinder and insert one end of the pipe in e and that we then lead our pipe around and e other end in a hole in the other end of the Suppose also our cylinder and pipe are I of water. If now we push the piston forward force the water from one end of the cylinder pipe and back into the other end of the cylinwe reverse the motion of the piston we will e water to flow back through the pipe in the direction. By pushing the piston back and e can thus make the current of the water flow ely in one direction and the other in the pipe. istration will serve well enough for our present to illustrate the difference between an alternatdirect current in electricity. In the case of an ing current of electricity we have a pressure or

E. M. F. acting along the wire first in one direction and then in the opposite direction and this pressure causes a flow of electricity or an electrical current first in one and then in a reverse direction. These reversals or alternations may take place very frequently. In ordinary alternating currents for electric lighting, the current alternates or changes its direction from seven thousand to fifteen thousand times a minute according to the speed and design of the dynamo. For most applications of electricity, it is best to use a direct current, as the laws governing the action of direct currents are very simple and thoroughly understood and machines for generating direct currents of electricity and for converting them into power have reached a high degree of perfection. There are however many applications of electricity for which alternating currents possess special advantages

Before we can obtain a clear idea of the object of using an alternating current, we must not only form an idea of electricity as a kind of motion which we can call a current, but we must consider it as a mode of transmitting energy. We transform the energy stored up in coal into energy stored up in steam. Then we lead the steam into an engine and transform the energy in the steam into energy in the form of mechanical motion. This energy in turn is applied to our dynamo and there transformed into electrical energy. Next the energy in the form of electricity is transmitted along a wire to some point of application, as for example an incandescent lamp, and there it is again transformed into energy in the form of heat and

wit

with a

sure

the

When our energy is stored up in steam we can the power given to our engine in terms of the and pressure of the steam. The greater the team to the engine, and the greater the preshe steam, the greater the horse power transIn a similar manner we can measure the Iven out by a dynamo to our circuit. The The pressure or E. M. F. and the greater the the greater is the power given out. In fact, t which work is done by electricity is not only nate to the pressure and to the current, but electrical units exactly equal to the pressure 1 by the current. We have seen that the unit re is the volt and the unit of the current is the The unit of the electrical power is the watt. at which the energy is being given out to is equal to the amperes multiplied by the volts, expressed in watts. For example: an ordinary andle power lamp requires a current of say in ampere and a pressure of 100 volts. The nsumed would be one-half multiplied by 100 ts. In electricity, as in steam, we can get the er by using a high or a low pressure, but if we pressure of the steam we can get our work steam, and if we lower the pressure we must steam to get the same amount of work. Just electricity. We can get a given amount of work from a dynamo giving a small current h pressure or a large current at a low pres

vided we increase the current as we decrease

and decrease the current

increase