MAGNETIC AND ELECTRIC CURRENT.

net pole) must necessarily yield currents alternating in direction.

(13.) By using a suitable commutator, all the currents, direct or inverse, produced during recession or approach, can be turned into the same direction in the wire that goes to supply currents to the external circuits; and if the rotating coils are properly grouped so that before the electromotive-force in one set has died down another set is coming into action, then it will be possible, by using an appropriate commutator, to combine their separate currents into one practically uniform

current.

(14.) To the moving conductor which is generating the electromotive-force by cutting the magnetic lines, it makes no difference what the origin of those lines is, whether from a permanent magnet of steel or from an electro-magnet, provided the number of magnet lines so cut is the same.

(15.) To the moving conductor it makes no difference what the origin of the motion is. Whether the motion be due to a steam engine, or to a gas engine, or to hand driving, or to the driving of an electric current in the wire itself (as in the case of electric motors), it makes no difference to the moving conductor, which provided that the speed and the number of magnet lines given will generate the same electromotive force.

ANIMAL ELECTRICITY.

Animal Electricity.-Several species of cretures inhabiting the water have the power of producing electric discharges by certain portions of their organism. The best known of these are the Torpedo, the Gymnotus, and the Silurus, found in the Nile and the Niger. The Electric Ray, of which there are

three species inhabiting the Mediterranean and Atlantic, is provided with an electric organ on the back of its head, as shown in illustration on this page. This organ consists of laminæ composed of polygonal cells to the number of 800 or 1000, or more, supplied with four large bundles of nerve fibres; the under surface of the fish is, the upper +. In the Surinam eel, the electric organ goes the whole length of the body along both sides. It is able to give a most terrible shock, and is a formidable antagonist when it has attained its full length of 5 or 6 feet.

To designate the charaeter of a current, and also a current with reference to its origin, various terms are used. Battery current, dynamo current, earth current, etc. These are terms used to designate the current from a battery, a dynamo or currents flowing through the earth on account of difference of potential at different points.

When a difference of electrical force or pressure exists in two places connected by a conductor, or a series of conducting bodies. a current will flow between the two points. The difference of force may be due to several causes, but whatever the cause the current will flow. The two places may be a few inches apart, as in a wire from a primary battery, or miles apart, as in a transmission system, or in the currents of the earth or air. They may be connected by a small copper wire, the rails of a street railway, or by a combination of a large number of conducting bodies. This conductor may be of large capacity, of low resistance, or may be a poor conducting medium.

Two kinds of current are generated, distinguished by the direction which they flow. The continuous or direct current

ELECTRIC CURRENT.

which flows continuously in one direction, and the alternating reversed current, which alternates the direction of its flow are the currents used. and these two kinds include a number of classes.

The alternating current may alternate the direction in which it flows ten thousand times a second, or twenty-five times a second; this is called the frequency of its alternations.

In

A constant current is an unvarying current. Although the voltage may vary the amount of current does not change. series arc lighting systems the current is universally constant. The quantity of electricity conveyed by a current is proportional to the current and the time it continnes to flow.

NOTE.-The telegraphers have made a series of tests for the purpose of ascertaining the actual amount of time which elapses while a signal is being flashed from America to Europe along the Atlantic cable. The tests referred to were made at the McGill University, Montreal, Canada, in June, 1891. In carrying out these experiments a duplex circuit was arranged on both land and sea along the entire line, which connects Montreal with Waterville, Ireland. When the line was "cleared" a chronograph was attached to the observatory wire at Montreal and everything declared in readiness. The instrument clicked off the signal, while the experimenters watched the chronograph with breathless interest. It did not seem to them "like an age of suspense," however, for within one and one-eighth seconds the chronograph recorded the return of the signal, while it slowly dawned upon the interested scientists present that the flash had actually made the round trip from Montreal to Ireland in a period of time but little greater than one-sixtieth of a minute. In that very short space of time, infinitesimal and almost unthinkable that electric message was flashed a distance almost as great as one-third the circumference of the world, or to be exact, 8,022 miles. Other experiments made the same day showed a variation of from one to 1.1 seconds for the signal to make a round trip.

ELECTRIC CURRENT.

The passage of a current through a conductor cannot be effected without a loss of power i. e, diminished pressure.

Loss of power means work done; if this work has a useful purpose it is not a loss in the common sense of the word, but in all other cases it is waste. Thus, any pressure of the current used in a lamp is no loss and nearly the whole pressure of the current, in a well designed installation, may be regarded as utilized in passing through the lamps, motors and other apparatus, a system shown.

The electric current, like steam and water flows in the direction of the least resistance. This is a general statement, to be modified by the fact that where two or more paths exist some current will pass through each. See page 79, "Divided Currents."

The speed of the electric current has been quite accurately determined to be identical with that of light.

Experiments have demonstrated the remarkable fact that alternating currents of a tension ten times that which is used in electro execution do not effect or injure the human body when passed through the same; and in fact are hardly perceptible in case the currents alternate 100,000 times in a second, that is, change their direction at this almost incomprehensible velocity.