for tramways, which must be placed under the floor of the car, field magnets of the Siemens type (see type of magnets, page 100) as shown in Fig. 96 are employed.

In dynamos of larger size the field magnets are so arranged as to present four poles to the armature, Fig. 94 ; two diametrically opposite poles being of the same sign. In this manner four distinct circuits through the armature are obtained, and correspondingly four brushes are used. If equi-potential points of the armature conductor

were permanently connected with each other, as has been done in the Elphinston-Vincent and in the Victoria dynamos, two brushes would suffice. The practical advantage of four poles over two is that double the current can be obtained without increasing the density of current in the armature conductor. On the other hand, there is a slight sacrifice of electro-motive force due to the greater magnetic resistance of air space and consequent weakening of the field. In cylinder armatures the area of each pole piece ( b of the formula given in Chapter IV.) must evidently be the smaller, the more separate pole pieces have to be placed round the armature, and, consequently,

the four-pole machine must have considerably more magnetic resistance than a two-pole machine of equal size.

The illustration, Fig. 95, shows the latest type of these dynamos, which are supplying current for the Blackpool Electric Tramway. They are each wound for a current of 180 amperes, and at 350 revolutions a minute their electro-motive force is 200 volts, which is found to be

sufficient for the working of the line. The field magnets are separately excited by a small dynamo of the type shown in Fig. 93, and the same size of armature, but in combination with Siemens field magnets, is used in the motors on the tramcars. Fig. 96 gives a perspective view of this motor.

The dynamo and motor employed by M. Marcel Deprez in his experiments on the transmission of energy, also belong to the class of machines under consideration, and should therefore be described in this place. But since the practical importance of these experiments renders it necessary to describe them in some detail, the author has