Small electric dredges are great where water power is available within reasonable distance, electricity may be generated and transmitted by cable to the dredge, proving an economical substitute for steam, especially in localities where fuel is dear. The small space, too, occupied by the plant, and the ease with which it is handled, make electricity a very convenient power for this class of work; and the necessary gear will compare favourably in weight, dimensions, and multiplicity of parts, with any steam plant.
The following description of the Sandhills dredge, on the Upper Shotover, is taken from the “ Lake Wakatipu Mail.” Power for the dredging plant, pumping, and lighting is supplied by two electric generators, driven by water power, two motors being placed on the dredge. The cost of the dredge was £7,000, of which £8oo was paid for freight and cartage. The generator station is 3 miles from where the dredger is working. This part of the plant consists of two Brush Victoria generators, driven by an ordinary 4-ft. Pelton wheel. The machines are capable of giving a total output of 52 kilowatts, or 70 h.p. The water for driving is brought from a creek by a 2-mile race. A pressure tank is placed above the generator station, and a line of pipes delivers the water at a pressure of 232 lbs. per square inch. The plant is equipped with a complete set of measuring and controlling apparatus, and connected with the dredge by telephone. The conductors conveying the power to the dredge are carried on patent insulators supported (2 ft. apart) on iron rails, the telephone wire occupying a central 
position immediately above. It is said that, notwithstanding the heavy current present in the power conductors, no inconvenience is caused, by induced currents, to the telephone line. To prevent damage to the telephone from accidental contact with the power line, an exceedingly fine safety fuse is included in the telephone circuit. The power is conveyed from the land lines by means of flexible cables suspended at convenient height above water. After passing through guide-blocks, the cables are led to revolving drums by means of a revolving connector attached to each drum. There is no interruption in the connections while paying out or taking in the cable. The two motors are duplicates of the generators, the electrical plant having been constructed specially for the work. The several parts of all the machines are interchangeable, and a spare armature and field magnet are provided in case of accident. A 16-in. centrifugal pump is worked by one motor, and the other drives the buckets, winches, and other gear in connection with dredging. All the necessary switches, shunt coils for controlling the machinery, are placed in convenient positions on deck. The pump, main shaft, and countershaft are driven by 5-in. rope bands, and have proved to be durable, efficient, and especially adapted to the work of the dredge. The dredge is lighted by two Brush arc lamps of 2,500 candle-power each.
A later illustration of an electrically-driven dredge is the Earnscleugh No. 3, designed by Mr. R. H. Postlethwaite, of the Risdon Iron & Locomotive Works, San Francisco. The main feature is the use of separate motors for every department. Thus there are six motors disposed as follows : One motor driving the bucket-chain, one driving screen and elevator, one operating the ladder winch, one working the head and side-line winches, one driving the sand-pump. The source of energy is the fall of the Fraser River. The pipe-line is about a mile and a-half long, the pipes being 2 ft. in diameter, the pressure at the power-house registering 172 lbs. to the square inch; here a three-phase alternator is driven by a turbine running at a speed of 400 revolutions per minute; the alternator is capable of generating 200 electrical horse-power. The continuous dynamo for exciting the field-magnets of the alternator is driven by a separate turbine; it is a four-pole shunt-wound dynamo. The conductors from the excitor and alternator are brought to the switchboard; those of the latter are provided with safety fuses; one of them is passed through an ampere-meter which measures the current, while a voltmeter is placed in parallel through a small transformer and measures the pressure. The potential difference between any two line-wires is 5,000 volts. The current then passes through the main switch to dredge by means of a pole-line about 3 miles long. On the dredge, the current passes through a transformer and is transformed from 5,000 volts down to 250 volts; the current then passes through the switch-board to the motor; the free ends of the motor coils are connected to a starting resistance. The motor has, at full load, an output of 100 h.-p.; a smaller motor for driving the winch has 25 h.-p. The current for the arc lamps and incandescent lights is passed through a smaller transformer which brings the pressure down to 50 volts.