60 .7374 = 44.244 ft. lbs.

Since one horse power = 33000 ft. lbs. per minute, the electrical equivalent of one horse power is

33000 44.244 = 746 watts.

or,

746 / 1000 = .746 kilowatts (K.W.)

Again, one kilowatt or 1000 watts is equivalent to

1000 746 = 1.34 horse power.

=The Farad.=--The measure constructed to hold a gallon of water may be called the gallon measure. The capacity of a condenser which would contain a charge of one coulomb under one volt pressure is the _farad._ It may seem strange that there is a unit of quant.i.ty and another of capacity to hold that quant.i.ty, when in the case of water the term "gallon" may suffice for the measure and the liquid it can hold. Electricity in this respect, however, corresponds to a _compressible fluid_ or a _gas_.

A gallon measure may hold a gallon of gas or ten; it depends entirely upon the pressure. So a condenser of a certain size may hold any number of coulombs, according to the electrical pressure.

The farad being inconveniently large for practical use, one-millionth of a farad, called a _microfarad_, is generally adopted.

CHAPTER VIII

EFFECTS OF THE CURRENT

The term "electric current," in the present state of our knowledge, should be regarded as denoting the existence of a state of things in which certain definite experimental effects are produced, for some of which there certainly is no a.n.a.logy exhibited in ordinary hydraulic currents.

The following are the most important of these effects:

1. Thermal effect; 2. Magnetic effect; 3. Chemical effect.

It is rather to these effects than to any imaginary current flow in the conductor that the mind of the reader should be directed.

With this preliminary caution, which should never be lost sight of, the use of familiar words and expressions connected with the flow of water in pipes is justified in order to avoid roundabout and c.u.mbrous phrases which, though perhaps more nearly in accord with present knowledge of the facts, would not tend to clearness or conciseness.

The three most important effects of the current just mentioned, may be presented in more detail as follows:

1. The _Thermal effect:_--

The conductor along which the current flows becomes heated. The rise of temperature may be small or great according to circ.u.mstances, but some heat is always produced.

2. The _Magnetic effect;_

The s.p.a.ce both outside and inside the substance of the conductor, but more especially the former, becomes a "magnetic field" in which delicately pivoted or suspended magnetic needles will take up definite positions and magnetic materials will become magnetized.

[Ill.u.s.tration: FIG. 87.--Lenz's apparatus for measuring the heat given off by an electric current. It consisted of a wide mouthed stoppered bottle fixed upside down, with its stopper, _b_ in a wooden box; the stopper was perforated so as to give pa.s.sage to two thick platinum wires, connected at one end with binding screws, _s_, while their free ends were provided with platinum cones by which the wires under investigation could be readily affixed; the vessel contained alcohol, the temperature of which was indicated by a thermometer fitted in a cork inserted in a hole made in the bottom of the vessel. The current is pa.s.sed through the platinum wires, and its strength measured by means of a galvanometer interposed in the circuit. By observing the increase of temperature in the thermometer in a given time and knowing the weight of the alcohol, the ma.s.s of the wire, the specific heat, and the calorimetric values of the vessel, and of the thermometer, compared with alcohol, the heating effect which is produced by the current in a given time can be calculated.]

3. The _Chemical effect;_--

If the conductor be a liquid which is a chemical compound of a certain cla.s.s called _electrolytes_, the liquid will be decomposed at the places where the current enters and leaves it.

=Thermal Effect.=--If a quant.i.ty of electricity were set flowing in a closed circuit and the latter offered no _resistance_, it would flow forever, just as a wagon set rolling along a circular railway would never stop if there were no _friction_.

[Ill.u.s.tration: FIG. 88.--The Seebeck effect: If in a complete metallic circuit having junctions of dissimilar metals, the junctions are at different temperatures, then generally a steady current will flow in the circuit as long as the differences of the temperatures of the junction is maintained. To demonstrate this, a piece of copper K bent in the shape seen in the figure, was placed on a block of bis.m.u.th A B, carrying a pivoted magnetic needle N S; as soon as the equality of temperatures was altered by either heating or cooling one of the junctions of the two metals, the needle indicated a current which continued to flow as long as the difference of temperature was maintained at the junctions. The movement of the needle indicated the direction in which the current flowed. If, for instance, the north junction B were heated, the N pole moved eastwards, showing that at the heated junction the current flows from the bis.m.u.th to the copper, at the cold junction from the copper to the bis.m.u.th.]

When matter in motion is stopped by friction, the energy of its motion is converted into heat by the friction thus causing the matter to come to rest. Similarly, when electricity in motion, that is, an electric current is stopped by resistance, the energy of its flow is transformed into heat by the resistance of the circuit.

If the terminals of a battery be joined by a short thick wire of low resistance, most of the heat will be developed in the battery, whereas, if a thin wire of high resistance be used it will become hot, while the battery itself will remain comparatively cool.

To investigate the development of heat by a current, Joule and Lenz used instruments on the principle of fig. 87, in which a thin wire joined to two stout conductors is enclosed within a gla.s.s vessel containing alcohol, into which is placed a thermometer. The resistance of the wire being known, its relation to the other resistances can be calculated. Joule found that the number of heat units developed in a conductor is proportional to:

1. The resistance; 2. The square of the current strength; 3. The time that the current lasts.

Joules' law may be stated as follows:

_The heat generated in a conductor by an electric current is proportional to the resistance of the conductor, the time during which the current flows, and the square of the strength of the current._

The quant.i.ty of heat in calories may be calculated by use of the equation,

calories per second = volts amperes .24. (1)

The total number of calories H developed in t seconds will be given by

H = P.D. C t .24. (2)

EXAMPLE--If a current of 10 amperes flows in a wire whose terminals are at a potential difference of 12 volts, how much heat will be developed in 5 minutes?

Subst.i.tuting in equation (2):

10 12 (60 5) .24 = 8640 calories.

Since by Ohm's Law potential difference = I R subst.i.tuting IR for P.D. in (2)

H = I^{2} R t .24

=Use of Heat from Electric Current.=--In the transmission of electricity from place to place, it is very desirable that none of the energy be expended in heating the conductor. Hence copper wires of the proper size must be used.

In wiring a building for electric lights, the insurance rules require that the wires be of a certain size and that they be put up in a certain manner. Otherwise they will not insure a building against fire.

It is often desirable, however, to use the electric current for the purpose of producing heat. The carbons of the arc and incandescent lamps are intensely heated that they may produce light. Coils of German silver wire or other high resistance wire are heated by the pa.s.sage of a current through them. In this manner the electric stove is made.

Soldering coppers, smoothing irons, and baking ovens are heated in a similar manner.

=Magnetic Effect.=--An electric current flowing in a wire causes it to be surrounded by a _magnetic field_, which consists of _lines of force_ encircling the wire. The field is strongest near the wire and diminishes gradually in strength at increasing distances therefrom. The presence of this magnetic field is shown by various experiments and the subject is fully explained in chapter IX on _magnetism_.

=Chemical Effect.=--Pats van Trostwyk (1789) pointed out that an electric discharge was capable of decomposing water; to show this he used gold wires, which he allowed to dip in water, connecting one of them with the inner, and another with the outer coating of a Leyden jar, and pa.s.sing the discharge through the water. The gas bubbles collected proved to consist of oxygen and hydrogen gas.

Nicholson and Carlisle (1800) dipped a copper wire which was connected with one of the poles of a voltaic pile into a drop of water, which happened to be on the plate connected with the other pole; gas bubbles appeared, and the drop of water became smaller and smaller.