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converters. However, the causes are now better understood and successful remedies are in sight. It may be mentioned that, while the French commission in their decision in favour of the con- tinuous-current system were largely influenced by the interference question, the Swedish commission regarded it as no better in this respect than the alternating-current single-phase system.

FIG. 2. 300-Ampere Mercury-Vapour Rectifier.

All three systems used for traction are capable of regenerative braking, by which is meant the use of the electric machine as a. generator absorbing the mechanical energy from the train and returning it to the supply system as electrical energy. In this re- spect the three-phase system is simplest, for all that is necessary here is that the speed should exceed synchronous speed, in which case the induction machines act as generators. Obviously the method is not suited for bringing trains to rest. With the other two systems special devices are requisite, and though regenerative braking was first developed for continuous-current traction, successful solutions have now been developed and applied on single-phase locomotives in Switzerland, which enable the train to be brought to rest by re- generative braking. Hitherto, in the matter of regenerative braking, economy in power has usually been of less importance than the sav- ing in wear and tear of tires, brake blocks and rails. On the lines

where such braking is applied, it is frequently impossible to utilize the returned energy, which is accordingly consumed in resistance.

Speed control can be obtained with all systems. With a continu- ous-current supply series, parallel connexion and field weakening together provide a limited number of economical running speeds. It must be borne in mind, however, that weakening the field reduces one of the torque-producing factors, which may entail serious in- crease in armature heating when the torque rises rapidly with the speed. With three-phase supply two or four speeds are obtained by cascade connexion or pole-changing devices. The single-phase system, by means of a variable-ratio transformer, provides most easily a large number of economical speeds.

Large mercury-vapour rectifiers have recently been constructed and put into commercial use ; these entail further auxiliary apparatus as vacuum and water pumps, and their relative advantage or dis- advantage as an alternative to the rotary converter for traction work remains to be decided in the future. Fig. 2 shows a small 3OO-ampere rectifier as made by Messrs. Power Rectifiers, Ltd., which can supply the rectified current at any voltage up to 750 volts. The arc operates in the lower chamber A, between the mercury cathode D and anodes C, of which there are usually six connected to the six-phase secondary of a transformer. The neutral point of the secondary is brought out and forms the negative pole of the con- tinuous-current system, the cathode being the positive pole of that system. The arc is struck by means of the ignition anode E, which is connected by a long rod with the solenoid mounted on the top of the condensing chamber B. This solenoid is controlled by a push-button ignition switch, and the connexions are so arranged that when the anode E touches the mercury a portion of the current which was previously flowing through the solenoid coil is diverted; this allows a spring acting in opposition to the solenoid to raise again the ignition anode. The rectifier is cooled by water circulated through the base of the cathode, through a jacket round the arc chamber, and thence through the plate in which the anodes are mounted and the jacket round the condensing chamber. Larger sizes dealing with 600 and 1,000 amperes are manufactured, and for larger outputs two or more rectifier cylinders are placed in par- allel and connected to a single transformer.

HYDRAULIC ELECTRIC STATIONS

Probably in no direction has greater progress been made of recent years than in the utilization of water-power. In all civilized countries throughout the world plants have been installed and projects drawn up for utilizing this natural source of energy. An idea of what is possible and of what has been done in this direction is obtained from the following approximate table, taken from a paper by E. M. Bergstrom (Inst. Mech. Eng. 1920): B.H.P.

Country

Available

Developed

Per Cent

U.S.A

28,100,000

7,000,000

24-9

Canada A ....

18,803,000

1,735.000

9-2

" B . . ..

8,094,000

1,725,000

21-3

Austria-Hungary

6,460,000

566,000

8-8

France .....

5,587,000

1,100,000

u-6

Norway ....

5,500,000

1,120,000

20-4

Spain .....

5,000,000

440,000

8-8

Sweden

4,500,000

704,000

15-6

Italy

4,000,000

976,000

24-4

Switzerland ....

2,000,000

511,000

25-5

Germany ....

1,425,000

618,100

43-4

Great Britain

963,000

80,000

8-3

Low, medium and high falls, ranging from 4 ft. (e.g. on the river Main) to 2,700 ft. of head (e.g. at Luchon on the French Pyrenees) have all been brought into service. To take one in- stance only, the modern water-power station on the river Dal, about 80 m. from Stockholm, contains four turbines, each of 10,000 H.P. coupled directly to dynamos at 125 revolutions per minute, and larger sets up to 20,000 H.P. are not uncommon. The latest (1920) station of the Southern Power Co., operating in S. Carolina, U.S.A., has been installed on the Wateree river for 90,000 H.P. and contains five turbines, directly coupled to generators each of 14,000 kva. The extension of station No. 3 of the Niagara Falls Power Co., developing an additional 100,000 H.P. at Niagara, is noteworthy for the inclusion of 32,500 kva. i2,ooo-volt three-phase alternators running at 150 revolutions per minute and a frequency of 25 cycles per second. One of these, manufactured by the Allis Chalmers Mfg. Co., is shown in fig. 3.

For high falls Pelton wheels are employed, and in the case of Luchon, quoted above, each Pelton wheel develops 6,200 H.P. at the high speed of 1,500 revolutions per minute. Still higher heads are being utilized, and owing to the high costs of material and