of asbestos slate S S are used to separate the layers of resistance strip. The trays are finally completely filled with sand. This serves to keep all parts rigidly in position, and materially assists in the dissipation of the heat. The cast-iron cover is cemented and bolted on to the tray One of these units, about 18 inches square, will successfully dissipate 10 H.P. for 10 minutes. The usual method of assembling them to form a rheostat is

to build up a number of them in one frame, connecting each one to a segment switch placed on the top or at one end of the frame.

of

Another example of a rheostat built up resistance units is that of the Electric Controller Co.'s reversible controller for crane motors, etc. Fig. 33 shows one of this company's standard resistance units. The resistance wire is wound on a heavy asbestos tube placed over a wrought-iron core. The rear end of the core is provided with a cap of insulating material C which is unaffected by heat, and which securely holds one end. of the resistance wire B. Two nuts F are also provided on this end of the core for clamping the parts of the unit in place. These nuts constitute one terminal of the unit. At the other end of the asbestos tube is placed a cap of copper D, which is electrically connected to the iron core, and is also adapted to receive and hold the other end of the resistance winding. By this method of construction both ends of the resistance winding are brought to the rear FIG. 29.-Controlling pillar of end of the coil, thus greatly simplifying the rheostat.

necessary connections.

The iron core is extended beyond the copper cap and is adapted to pass through the slate face of the controller, where it is held in place by a locknut working in a countersunk recess in the slate. The contact button, or segment, screws directly on the end of the iron core above the lock-nut. These buttons have octagonal heads, which may be easily grasped with a wrench. It will be seen that in this way a button may be removed and replaced without disturbing the resistance unit and its connections. will also be observed that the turns of resistance wire form the exciting winding of an electro-magnet, of which the iron rod at the centre of the coil forms the core. When current passes through the coil, the core

becomes magnetised with one pole at the centre of the contact button. This places the contact button in a powerful magnetic field, so that any

current is heavy and the danger of arcing increased, the magnetic field is

stronger, thus adjusting itself to the demands upon it. This blow-out

feature is secured without complicating or adding to the size of the controller.

A somewhat unusual type of rheostat is to be seen in the Paderno generating station near Milan, for regulating the fields of the generators. The resistance wires and strips are built up somewhat on the lines of a squirrel cage, mounted on a shaft and provided at one end with a commutator to which the various sections of the rheostat are connected. Fixed brushes making contact with the commutator are connected in series with the field circuit. To cut resistance in or out, the entire rheostat is rotated by an extension to the shaft, terminating in a handle on the switchboard.

CHAPTER III.

CIRCUIT-BREAKERS, OR CURRENT-INTERRUPTING DEVICES.

Various methods of breaking an are:-Quick break, Carbon break, Water break, Magnetic blow out, Shutter break, Oil break, Multiple break, etc.-Prof. Hopkinson's experiments-Respective functions of manual, mechanically operated, and fusible circuit-breakers-Field circuit-breakers, constructed to insert resistance or shortcircuit field on opening-Examples of quick break circuit-breakers: Mordey,' 'Westinghouse,' etc. Examples of water break circuit-breakers: Raworth,' 'Cowan,' 'Brush,' etc.-Examples of blow-out circuit-breakers: Fowler,'' Bates,' 'Schuckert,' ' Stanley,' ' Cowan,' etc.—Horn break circuit-breakers-Experiments to show that their action is not due to heated air-Theory explained-Modified arrangement of horns-Blow-pipe action of horn break fuse-Liability to induce surgings in high-tension cables-Carbon-tipped horn break circuit-breaker-'Siemens' plunger circuit-breaker-'Partridge' vacuum circuit-breaker-' Partridge' sparklet fuseExamples of oil break circuit-breakers: Ferranti' H.T. oil fuse, 'Ferranti' extra H.T. multiple oil fuse-'Ferranti,' 'Cowan,' and 'Stanley' oil break switches'Schuckert' and 'Parshall' multiple break circuit-breakers Shutter circuitbreaker-Mordey' dust fuse-Shunted circuit-breakers.

THE term circuit-breaker is here used to signify a device for interrupting a current as distinct from a switch, the use of which is assumed to be confined to directing the flow of current.

Reference has been made in a previous chapter to the difficulties which arise in breaking a circuit carrying a heavy current. These difficulties are proportionally greater in dealing with high-tension currents. A pressure which is insufficient to cause the current to spark across an air gap an eighth of an inch wide is sufficient to maintain the circuit across a gap two or three feet wide when a heavy arc is established. This is chiefly due to the fact that the arc itself becomes a moderately good conductor, owing to the presence of metallic vapour caused by the action of the intense heat produced immediately the arc is established, on the metal

contacts.

Various devices have been used with more or less success for overcoming these difficulties. They practically all depend, however, upon one of the following principles:

(A) The formation of an arc is prevented by very rapid separation of the contacts, thus increasing the gap between the points of contact to a