of a switch board usually carries a ments, switches and safety devices, so it is desirable to place our conductof the board. In a large plant there t many instruments and appliances, ons of the switch board are limited unt of space available and by the nvenient operation, it is almost absoO place the conductors behind the wires were placed behind the board cheap work could be done.

x of a switch board should be as neat = that upon the front, and this is a ility and the intent of the installing

rule "b" should mean not only that ust be readily accessible for inspecx there must be ample room behind for a man to work and to do good sential, as it is often necessary for a d a board while the conductors are r, as it is commonly expressed, while wing "live" circuits. When a board a brick wall, all wiring should of e face of the board, and it should not ttach conductors of any kind to the crews or bolts extending through the

witch board construction are allowaWe may make our board entirely of marble fastened to metal supports; or

y erect a wooden framing attaching to the front struments, switches and controlling devices, each ce having its slate or marble base, and the cons being supported on the back of the frame upon ›r porcelain insulators. Frame or "skeleton " iction may be made safe, but it requires more id skill to do a neat-looking job of wiring upon a n board than upon one of slate or marble, and te or marble board costs but little more than the which are required when a "skeleton" board is The superior appearance of a marble board justifies the slight additional expense. Marble er than slate as an insulator, and but slightly expensive. The purer the marble, the better; here the switch board carries currents of very ressures, the marble should be carefully selected, ould be free from any impurities that might its insulating qualities.

These coils

stance Boxes and Equalizers.-A Resistance Box
eostat is a regulating device introduced into an
è circuit for the purpose of reducing the current
electro-motive force. It is simply, as its name
es, a resistance. The earliest form in common
nsisted of coils of wire of a poorly conducting
al, such as iron or German silver.
mounted in a box, and the top of the box carried
h, by means of which more or fewer coils could
-oduced into the circuit, so that by turning a
or wheel the resistance of a circuit could be
When we add resistance to a circuit, the
F. or pressure which tends to send a current

or motors is decreased, and if the emains unchanged, the current will tely decreased. A rheostat correle on a steam engine. When a rhetrolling a current in the magnet or I dynamo, it is called a "Dynamo d Regulator," or simply a "Resistt is placed in the circuit of a motor rting Box" or "Rheostat." When co reduce the pressure upon a numing a common starting point, they ers." Where a rheostat is used in "turning down the lights," it is The most general term is "Rheoto any resistance that can be varied

f a current by inserting resistance of energy. This energy is transthe rheostat. In order to make a gh to be used in commercial work, low the wires to get pretty hot. It that rheostats should be carefully led. Perhaps the worst points of the central stations of the past were ade of coils of iron wire in wooden may be made perfectly safe by makof non-inflammable material, and mable material from their vicinity. of heat, and should be treated as

ters.-A lightning arrester is not a

device to arrest lightning, although that is "what that name might imply." Lightning, when once it gets upon our wires, is always looking for a chance to get to earth, and a lightning arrester is a device intended to assist the lightning to get to the earth by an easy path, and thus prevent it from taking a path where it might cause fire or other damage. A lightning discharge has a sufficiently high electrical pressure to overcome the insulation of our wire. In fact, it will get to ground somewhere, even if it has to jump a considerable distance to the earth. It will take the easiest path. Without proper protection, this path may be through the insulation of our dynamo (especially if the iron frame of the dynamo is grounded), or it may be from our wire to a gas pipe in a fixture carrying both gas and electric lights, or it may be at any weak point in the insulation of our conductors from the earth.

Most forms of lightning arresters consist simply of a piece of conducting material brought close to another piece of conducting material, so that the two are separated only by a very small air gap. The most simple form consists of two rectangular plates of brass or carbon laid upon slate or marble so that their edges nearly touch, one plate being connected to our circuit which is to be protected, and the other to the earth. The adjacent edges are usually notched, as this assists the discharge. Although the E. M. F. of our dynamos will not cause a current to pass across a very small air gap, the E. M. F. of a lightning discharge is so high that the gap offers no apparent resistance to its passage. Such is the nature of electricity in this form that it may jump

an air gap of considerable width, rather than pierce a comparatively poor insulation at another point not far away upon the same circuit.

The great danger from lightning is that it destroys our insulation. We have seen that although the pressure of an ordinary dynamo will not cause a current to pass over a very minute air gap, still when we touch two wires of opposité polarity together and then separate them the same pressure will permanently maintain an arc of considerable length. So when lightning passes to ground across an air gap it may start an arc which will be maintained by our dynamo with disastrous results. The entire theory of lightning arresters is too long to discuss in this volume, but the principles of protection from lightning are pretty well understood, and except in certain localities we can secure ample protection by using any one of a number of commercial forms of lightning arrester, provided we use enough of them and have them well distributed over our system of over-head conductors.

66

Lightning arresters are of course not needed in an isolated plant not having out-door conductors. Rule calls for an arrester on each side of every circuit. This is necessary, for if arresters were placed only upon one side (that is attached to only one pole of our circuit) the lightning would have to go through our dynamo or our lamps or motors before it could get from the other pole to an arrester.

As regards Rule "b," everyone knows that lightning moves in mysterious ways, that while its action lasts but an instant it may leave destruction in its path; and