Even a pretty thick layer of oxide will be reduced and drop off, while smaller quantities of oxide unite to form a slag with the sandy clay frequently added as a flux. This slag prevents the oxidation of the metals while cooling. No other fluxes are required. The operations can also be carried on under water, although the gases and steam generated cause trouble. Nevertheless, an apparatus has been constructed to facilitate such work by forcing the water away from the parts to be treated by means of compressed air. One of the chief advantages claimed for the new system appears to be that the arc is brought to the work, and not the work taken to the arc, which would mean transformers, crucibles, or other apparatus. Size is hence a question of secondary importance, and unwieldy pieces may be dealt with, although for soldering work of the ordinary kind a special operating table is employed as more convenient. An accident at Struve's emery works, St. Petersburg, directed general attention to this process. The works have a vertical boiler with heaters, some of which had become leaky, and the works were practically at a standstill. The consulting engineer declared that the repairs would be rather expensive, and might occupy 3 weeks' time.

Benardos inspected the boiler, and offered to repair the heaters that very day. The boiler was put on a truck, taken to his works, treated electrically on the truck, and wheeled back, all in 3 hours. Prof. Ruhlman saw the boiler in full action the next working day. Fig. 148 is reproduced from a photograph taken during these repairs, and illustrates the simplicity of the process. We may mention another case reported by Prof.

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Ruhlmann. A cast-iron flywheel of more than 5 tons weight had been broken into several pieces while being taken down from the truck. The pieces were fused together within a few hours, and the following day the flywheel was in place and at work.

A glance at Figs. 143-50 will give an idea of the great variety of circumstances under which this process is applicable. It is clear that flywheels cannot be treated in the same way as telegraph wires; and that a soldering and welding plant, to be really useful in the workshop or foundry, should be able to deal with delicate articles of a few lb. or oz. in weight equally well as with heavy pieces. Economy will in general be in favour of one source of power for the various operations; but then the operator must have thorough command over the volts and amperes of his currents if the arc is to have the proper volume and temperature. The length of the arc may, within small limits, be adjusted at will; the currents themselves may be modified with the help of resistance coils. But this is not sufficient. Supposing that the workman has to do a little tin soldering, and to weld two large thick boiler plates a few minutes later, he must be in a position to vary both tension and quantity of the currents within very wide limits. A dynamo alone would not do, there must be accumulators also, and these of a special kind capable of being charged with strong currents and discharged either at a few amperes or at several hundred times that amount.

Faure accumulators are not adapted for such work, nor are those of the Plante type, as they cannot store up sufficient quantities of electricity, although they bear strong charging and discharging currents. Benardos has constructed accumulators for the work which are not strikingly novel, but seem well fitted for their special purpose. Prof. Ruhlmann saw at St. Petersburg some cells in very fair condition which had been in use for more than 1 1/2 year. It is further noteworthy that at the Creil works, where the Benardos processes 5 have been under trial for some time, serious difficulties had to, be encountered until the accumulators already, there were exchanged for the Benardos battery. The complete cell (Fig. 143) weighs 35 lb. and contains 9 lead plates (Fig. 144), all of the same kind, 4 positive and 5 negative, with 1 1/2 sq. yd. total surface. Each plate consists of a frame cast of pure lead, 6 in. by 7 3/4 in. surface, and 1/5 in. thick. The interior of the frame is filled with strips of thin lead, alternately straight and corrugated (Fig. 145), soldered into their places; the latter strips are bent in such a manner as to facilitate upward currents in the liquid.

Such currents arise during charging, owing to the development of gas, which, if kept within proper limits, is thought advantageous to these cells; the upward currents equalise the difference in density; the curvature of the bent strips favours the liberation of the small gas bubbles, and checks the formation of larger bubbles, which would cause buckling. Caoutchouc prisms separate the positive and negative plates, the poles of which are simply soldered to stronger lead strips running along either side of the cell. Diluted sulphuric acid of 1.25 sp. gr. circulates freely between the plates. The total weight of the complete cell (35 lb.) is made up of 24 lb. for the 9 plates, 8 lb. for the acid, and 3 lb. for the glass jars. The cells have ah interior resistance of 0.002 ohm and give 2.5 volts, when continually charged while at work; 50 to 70 of these cells are joined in a battery; several batteries, 3 for instance, are grouped in parallel, and are continually charged by a shunt dynamo. Fig. 146 explains the ordinary connections.

The shunt dynamo charges the 50 accumulator cells in series; a voltmeter and an amperometer are inserted at V and A. From the positive terminal of every fifth cell a wire leads to a plug switchboard U; from U the current passes through a variable resistance W, and from thence through a flexible cable to the carbon holder Z and the carbon pencil K. The operator manipulates his holder Z, the metal to be fused, placed upon the table P, being joined directly to the negative terminal of the battery. By inserting the plug in the switchboard U, the operator may obtain currents from 5 cells, twice 5, and so on to 10 times 5 cells. If considerable masses of metal are to be dealt with, currents of considerable strength are needed. These are obtained by grouping the batteries or certain sets of cells in parallel. Supposing the dynamo gives currents of 175 volts and 120 amperes, that there is a battery of 70 cells coupled in series, and that it is desired to solder two boiler places 2/5 in. thick. The carbon holder is connected with the positive terminals of the fortieth cells of 3 groups. The carbon pencil is allowed to touch for a fraction of a second, and is taken off again immediately, so that between the plates and the carl*on pencil an arc of a few millimetres length is formed.