an outer ring; a rim which is grasped by jaws; or two cones that are forced together. The force is obtained by screws or toggle joints. Fig. 79 shows one type in which jaws carried by the shaft are caused to grasp a rim cast upon the pulley, the latter being mounted to turn freely upon the shaft when the clutch is released. The outer jaw on one side is carried by the same arm as the inner jaw on the other side; hence the grip is obtained by moving one arm upward and the other downward by means of the toggle operated by the collar on the left, which, in turn, is caused to slide on the shaft by a fork and lever controlled by hand. If used as a simple friction clutch to connect two shafts, no pulley is required, the rim being rigidly mounted upon one shaft and the jaws upon the other. The jaws are adjustable and are lined with renewable pieces of maple.

Magnetic Clutches.-The substitution of magnetic attraction for ordinary mechanical force, to obtain the friction required in a clutch, secures several decided advantages. The most important is the fact that the pressure is self-contained, the attraction being exerted only between the friction surfaces, without the end thrust or external force involved in all mechanical clutches. Complication of levers and pivots is avoided, the parts being few and simple. A magnetic clutch can be controlled from a distance and at several points. It is lighter and occupies less space for a given power transmitted. The form devised by Mr. B. J. Arnold, represented in Fig. 80, consists of two cast-steel rings A and B carried on steel webs E and F which are bolted to the hubs G and H fixed on the shafts J and K to be coupled. The magnetizing coil M is located in an annular slot in the field ring A, and is supplied with current through insulated rings on G. A brush-holder (not shown) is attached to the floor, wall, or ceiling. The armature B is separated to inch from the field A, according to size, when no magnetizing current flows. But the elasticity of the webs E and F allow the field and armature to spring together when the circuit is closed, the magnetic attraction holding the two in contact with great force. An Arnold clutch 48 inches in diameter will transmit 228 H.P. at 100

16

r.p.m. and consumes 2.58 amperes at 110 volts or 284 watts, being Several large electrical plants in Chicago,

less than of 1 per cent.

St. Louis, and other places employ these clutches to connect the various engines and generators.

CHAPTER XVI.

PRINCIPLES OF DYNAMO-ELECTRIC MACHINES.

Introduction. The history of these machines has already been given in the general history of electric lighting in Chapter II.

A dynamo-electric generator is a machine for converting mechanical energy into electrical energy by causing conductors to move in a magnetic field, or vice versa. These machines operate according to the principle of magneto-electric induction discovered by Faraday in 1831.

Before taking up the consideration of the machines themselves, it is proper to give the principles of electro-magnetism, which play an important part in their construction and action.

Magnetic Field. If an ordinary permanent magnet be approached by a small piece of iron or steel suspended on a thread, it will be attracted by the magnet. The space near the magnet in which this phenomenon takes place is called the magnetic field, or simply the "field." This region has no definite limits, since the distance at which magnetic effects can be obtained depends upon the sensitiveness of the instrument used. Delicate galvanometer needles, for example, may be deflected perceptibly at a distance of several hundred feet from the field-magnet of a large dynamo. It is customary, however, to consider the field as being confined to the immediate neighborhood of the magnet where the effect is strong. The extremities of a magnet usually exhibit magnetic properties more powerfully than the middle portions, and are called the poles. Magnetism is commonly regarded as consisting of a number of "lines of force" or "tubes of force." This conception was suggested by the lines in which iron filings arrange themselves under the influence of a magnet, the direction and intensity of the field being represented by the direction and number of the lines. This idea is often very convenient, but should not be taken too literally, as it sometimes leads to wrong notions.

The Magnetic Circuit.-To localize and strengthen the magnetic field, as well as to economize the wire and current required, the

B

C

B B

magnets of generators or other electromagnetic apparatus are arranged in such a form that the path of the lines is as nearly as possible a closed circuit; that is, for the most part in iron, only a sufficient air-gap being left for the inductors and the clearance which they require for free motion. In Fig. 81 the general form of the magnetic circuit of a bipolar dynamo is shown, CC being the magnetic cores, upon which are wound or placed the coils or bobbins of copper wire BB through which the exciting current flows. Y is the yoke which connects the two cores, and forms with them the "horseshoe" type of magnet. The extremities of the magnet are provided with pole-pieces PP, which are bored out or shaped to receive the armature, which consists of the armature core a and armature conductors or "winding" W.

Fig. 81. Magnetic Circuit of
Simple Dynamo.

Various other forms of magnet are used, but in almost every case they are equivalent to the horseshoe form shown; and even with multipolar field-magnets, that is, those having more than two poles, it is usually possible to consider them as being made up of several horse-shoe magnets, or, in other words, two or more magnetic circuits.

Calculations in regard to the magnetic circuit are usually made by means of a formula analogous to Ohm's law for the electric circuit, this expression being:

There is, however, one important difference between this formula and Ohm's law; the reluctance is not a constant or an independent quantity like electrical resistance. It depends upon the value of the flux, or rather upon the flux density. Hence it is necessary to know the number of lines of force per square centimeter in the iron, in order to fix the value of the reluctance; whereas electrical resist

ance has a constant value, and is independent of the strength of current, provided the temperature does not change. Furthermore, magnetic leakage is usually a much larger factor, and more difficult to determine, than electrical leakage, see data, pages 308 to 319.

Magnetic Flux.-The total quantity of "induction" or field in a magnetic circuit is called the flux, and is measured in lines of force. The term "line of force" was originally used by Faraday in a general sense to express the direction and intensity of magnetism in something the same way that the expression "ray of light" is used. When magnetism came to be measured definitely, the line of force was adopted as the unit of magnetic field or flux. If we follow the derivation of the value of the line of force, we find that a unit magnetic field exists at the distance of one centimeter from a unit magnetic pole, the latter being a pole of such strength that it repels a pole of equal strength with a force of one dyne at a distance of one centimeter. Each square centimeter of the surface of an imaginary sphere with one centimeter radius described about a unit pole will contain a flux of one line of force; and since the total surface of the sphere is 47, it follows that 47, or 12.57 lines, emanate from a unit pole. This somewhat indirect definition is made much shorter and more convenient by simply stating that one hundred million lines of force cut per second generate one volt E.M.F. Since the values of the volt and other electrical units have been defined and legalized internationally, it is proper to base the magnetic units directly upon the electrical ones. Very frequently, instead of considering the total flux we treat the flux density, or intensity of magnetic field; that is, the number of lines of force per square centimeter. This quantity multiplied by the area of the field or cross-section of the magnet gives the total flux. The maximum flux density commonly used in practice is from 14,000 to 16,000 lines of force per square centimeter (90,300 to 103,200 per square inch) for wrought iron, and from 6,000 to 7,000, or about one-half as much, for cast iron. These values are what are called "practical saturation," beyond which it is not ordinarily economical to go. As a matter of fact, however, Ewing and other experimenters have forced magnetic density as high as 43,000 lines of force per square centimeter. On the other hand, in alternating-current transformers the ordinary flux density has a maximum value of only 4,000