should be maintained between the conductors and the cores or frame. This standard is usually put at one megohm for lowtension machines of about 100 volts, and correspondingly higher for greater E.M.F.

Besides the mere insulation resistance, it is necessary to consider the point at which the insulation will break down entirely. A test might show, for example, an insulation resistance of 10 megohms, but it might be punctured and destroyed if 500 volts were applied to it. It is therefore necessary to make a "breakdown test" also, at a potential from two to five times the voltage for which the machine is intended.

The voltage required to break down cotton and silk covered wires, fibre, mica, etc., has been investigated by Canfield and Robinson; the data in regard to the latter materials have also been determined by Steinmetz † and others.

The effect of moisture on the insulation resistance has been studied by T. T. P. Luquer, ‡ and the effect of temperature by F. C. Reeve. §

* Electrical Engineer, March 28, 1894.

† Trans. Amer. Inst. Elec. Eng., vol. x., p. 85, 1893.

Electrical Engineer, Dec. 28, 1892.

§ Electric Power (N.Y.), June, 1895.

CHAPTER XVIII.

TYPICAL FORMS OF DYNAMO FOR ELECTRIC LIGHTING.

THE types of dynamo used in electric lighting are so numerous, and are modified so frequently, that it is useless to attempt to describe all of them. Moreover, the principles laid down in the preceding chapter are intended to enable one to understand and judge any particular dynamo on its merits. An examination of the machines themselves is by far the best way to compare them; and next to that the most definite and complete information regarding the different forms can be obtained from the catalogues of the various manufacturers.

Certain types, however, are so important and interesting, that they deserve a description which will also be of assistance in studying other forms.

The dynamos used in electric lighting may be divided into the following classes:

A. Direct-Current Machines.

1. Bipolar, constant-potential for incandescent (and arc) lighting. 2. Multipolar, constant-potential for incandescent (and arc) light

ing.

3. Closed-coil, constant-current for arc lighting.

4. Open-coil, constant-current for arc lighting.

B. Alternating-Current Machines.

5. Constant-potential for incandescent (and arc) lighting. 6. Constant-current for arc lighting.

DIRECT-CURRENT, CONSTANT-POTENTIAL DYNAMOS.

This is the most common form of dynamo, since it is used for incandescent lighting in nearly every isolated plant and in a majority of the central stations. The current generated by it is also suitable for constant-potential arc lighting, electric motors,

electric heating and cooking apparatus, storage batteries, electrochemical and electrometallurgical purposes, etc.

The armatures of these machines are provided with closed-coil windings of either the drum or ring type. The field-magnets are usually either shunt or compound wound, but in some cases are separately excited. They are bipolar or multipolar in form, resulting in considerable differences in design and appearance; the former will be considered first.

The Edison Dynamo is one of the oldest and most prominent examples of this class. It consists, as shown in Fig. 112, of a horseshoe (undertype) field-magnet, and a drum armature. The pole-pieces are magnetically separated from the base by zinc castings.

This type was formerly employed exclusively in all stations and isolated plants using the Edison system, but is now being replaced to a great extent by multipolar machines for direct coupling with engines, and also for belt-connection. Nevertheless, at the present time there are probably more of these machines in use for direct-current generation than of any other one form. They are simple, substantial, and symmetrical in appearance; but considerable material can be saved by adopting some more modern design.

The Edison-Hopkinson Dynamo is the Edison type as designed by Hopkinson, and manufactured by Mather & Platt of Manchester, England. The chief modification is in the form of crosssection of the field-cores, which is usually rectangular. But this is a very doubtful advantage over the circular cross-section, as explained in discussing field-cores on page 302. Hopkinson, however, introduced the great improvement of shortening the fieldmagnets, and making them of larger cross-section, in place of the long small-diameter cores of the early Edison dynamos. He also showed that the construction of the first large Edison dynamos, with several cores side by side, united to a common pole-piece, was very wasteful of material, since one core of the same total

cross-section would require much less wire. In fact, almost the entire theory of the magnetic circuit, as applied to the design of dynamos and motors, is due to Hopkinson, whose methods are followed to-day.

The Thomson-Houston "Motor Type " Dynamo embodies a horseshoe field-magnet of the overtype. The armature is of the drum form; and since it has a smooth core (i.e., without teeth),

the large number of ampere turns required in the field necessitate long cores, which bring the armature quite high above the base; otherwise this type is similar in general form to that shown in Fig. 113. This type of machine was introduced to replace the "squirrel-cage" field-magnet with spherical armature used in the first Thomson-Houston incandescent lighting machines, which latter had the same general appearance as the arc dynamo (Fig. 119). The name is derived from the fact that the design.

was first applied to motors. This type, like the original Edison, is being replaced by the more modern multipolar dynamos of the General Electric Company.

The Crocker-Wheeler Bipolar Dynamo, shown in Fig. 113, is similar in general form to the preceding; but the length of fieldcoils, and consequently the height of the armature-shaft above the base, are reduced by using a toothed armature, which requires a much smaller number of ampere turns in the field-winding. The armature is of ring form, and therefore relatively larger in diameter than the drum armatures of the two types of dynamo already described. The field-magnets are either drop-forgings of

These

wrought iron, or castings of steel set into a cast-iron base. machines are made in sizes of 1, 1, 1, 1, 2, 3, 5, 7, and 10 kilowatts capacity, with speeds ranging from 2,000 to 1,000 revolutions per minute respectively. Larger sizes have multipolar

field-magnets.

The C. & C. Dynamo is a well-known form, the field-magnet of which is of the Manchester or consequent pole type, having a double magnetic circuit, as illustrated in Fig. 114; the axis of the cores being curved.

The Mather Dynamo is another machine in which the core is curved; in fact, it forms almost a complete ring, as shown in Fig. 115. The objections to field-magnets of these shapes are the