Section "d" is a regulation that is especially demanded by insurance companies. Many systems of wiring in buildings, as for example telephone, telegraph and clock circuits, use ground returns and the connection between such wires and a trolley wire furnishes a path to ground within buildings and over conductors not insulated for high pressures or large enough to stand strong currents. Such connections or "crosses " have caused some very serious fires. Safety is usually sought by suspending one or more guard wires over each trolley wire. The guard wires are light iron wires whose function it is to support any fallen wires of other systems so that they cannot touch the uninsulated overhead railway conductors. Car wiring comprises the conductor from the trolleys to the motors and controlling devices and to the incandescent lamps in the car. The preceding remarks about the pressure of street railway circuits and the consequent danger from shocks sufficiently explains Rule 40.

Rule 41 refers to the use of street railway circuits for furnishing electric lights and power in buildings. This use is prohibited except in power houses and car barns. There are many obvious reasons for this. Where a "ground" return is used it is of course impossible to insulate the conductor from the ground. One side of the system is "grounded" and the other side is separated from the ground by only one thickness of insulation. The use of such circuits in buildings, is a hazard to property and a menace to life. There are many reasons why the practice of using street railway circuits for lighting and power in buildings is bad and it is

o allow such applications when high insulation aded upon low potential lighting or power cirOwing to the varying pressure on street railway their use for lighting is not often desirable, vere permitted they would be extensively used ating stationary motors, provided it was safe to ut it is not safe except in a few isolated excepses, where special precaution can be taken, as ple in a fire-proof building into which the wires ht directly from street. In general where there g to burn, the use of a 500 volt grounded cirry improper.

3 has doubtless been called forth by the xperienced by electrolysis. We have spoken rent as returning to the station by way of the its path is not necessarily confined to the the rails are laid in the ground, the earth s a return path and in fact it was once thought to depend on the earth as a part of the return The current will return along any path that is If there are iron waterpipes, or gas pipes or red cables in the vicinity of the rails, a pore current will flow back on these or along any allic path. The term "electrolysis," as used tion with street railway circuits, refers to an electric current similar to that which takes n electricity is used for electro-plating. When flows from a rail to an iron pipe, then along nd back to another rail, if the rails and pipes. amp place electrolysis will take place; that is

ried off with the current at the point where it leaves and the rail or pipe will be eaten away, as a metal plate is eaten away in an electric battery or in a plating bath. The greater the current the greater will be the amount of iron carried away in a given time. The result of this action is the rapid pitting and corrosion of water and gas mains by the current of electric street railways using a ground return. There are two remedies for electrolysis: First, we may use an overhead instead of a rail return for the current. This is very expensive and not very practical, as it requires twice as many overhead wires and makes very complicated and unsightly overhead construction, especially crossings and turn-outs. The whole problem is too long to discuss fully here. Electric railways may cause a hazard by eating through gas mains and igniting the gas, or by destroying the insulation of underground electric cables leading into buildings.

Rule 43 is apparently an attempt to limit the damage from electrolysis. By limiting the drop in volts (or the loss in our return circuit) we may decrease the danger of the current straying from rails to pipe, etc., and thus lessen the danger of interference with other systems. But, in the light of recent experience, the enforcing of Rule 43 would not go far toward removing these troubles. The loss of pressure (or difference of potential), allowed by Rule 43, is too great to render the rule of any value, but it is to be hoped that, at no distant date, legislation or the action of the courts will demand some remedy that will prevent the damage to property which is now going on all over the country.

CHAPTER XVIII.

CLASS F, STORAGE OR PRIMARY BATTERIES.

TEXT OF THE CODE COVERED BY THIS CHAPTER. CLASS F. 44. STORAGE OR PRIMARY Batteries:-a. When current for light and power is taken from primary or secondary batteries, the same general regulations must be observed as applied to similar apparatus fed from dynamo generators developing the same difference of potential. b. All secondary batteries must be mounted on approved insulators. c. Special attention is directed to the rules for rooms where acid fumes exist. d. The use of any metal liable to corrosion must be avoided in connections of secondary batteries.

DEFINITION. RULE 44. STORAGE OR PRIMARY BATTERIES:Section b. Insulators for mounting secondary batteries to be approved must be non-combustible, such as glass, or thoroughly vitrified and glazed porcelain.

The practical difference between a primary and a storage battery is pretty well indicated by the terms. "Storage" and "Primary." A primary battery, like a dynamo, is a source of electrical energy. In fact, it can more properly be called a source of electrical energy than a dynamo, as the current is generated in a primary battery without the application of any external force. A storage battery, on the other hand, is simply a device by which the electric energy generated in a dynamo or primary battery is stored. The analogy may be a little far fetched, but we may use our com

parison between a current of water and a current of electricity by saying that a dynamo corresponds to a pump driven by some prime mover, a primary battery corresponds to a chemical fire engine, which will maintain a flow of water as long as the supply of chemicals is maintained, and a storage battery corresponds to an elevated tank, out of which we can get a flow of water only after it has been filled up by our pump. The difference between a primary and a secondary battery does not have any special bearing upon the rules concerning their installation and care, for two reasons: first, a primary battery is such an expensive source of obtaining electricity that it is never used for producing electricity for light and power; again, if primary batteries were so used the rules regulating their installation and use would be the same as for storage batteries.

It may, however, be of passing interest to describe a little more fully the difference between the two kinds of batteries. If we take two plates of two dissimilar metals and immerse them (without their coming into contact with one another) in a liquid which has a chemical affinity for either one of the metals, we have a primary cell. If we connect the two plates outside of the liquid with a wire or other conductor, an electric current will flow through the circuit thus formed, i. e., it will flow from plate number one to plate number two through the liquid within the cell, and from plate number two to plate number one through the wire outside the cell. The pressure or electro-motive force of the cell will depend upon the metals selected for the plates. For any particular cell the strength of current which it