If the resistance be great, a small current will be required, but the pressure of the current will need to be high enough to force the cnrrent to pass, according to Ohm's law, E. M. F. or pressure Current = • Resist an ce But if the resistance in the circuit be low, the current, to effect heating, will require to be increased, while the pressure, or electromotive force, will be less. It is of course evident that in the case of two bars or pieces of metal held together firmly, and arranged so that a cur rent when passed will only go through the pieces at the meeting portions, and a little of the metal each side thereof, a very low resistance will exist in the path of the current through the pieces. Hence the desired heating for welding will demand that the current strength or rate of flow of electricity be very high. This current, with bars of copper up to 1 in. or a little more in diameter, or with bars of iron of seven or eight square inches of section at the weld, may reach thirty or forty thousand amperes, yet the pressure or potential difference causing such flow may be no more than two volts. In actual practice in electric welding, the strength of current and pressure depend on the conditions of the work, the rate of heat development, and other factors. It therefore varies greatly. For obtaining the large currents at the low pressure indicated above, the development. of the art has shown that storage cells or accumulators may be used, that dynamo-electric machines may be constructed to furnish the currents, or that currents of comparatively high pressure and small flow may be transformed or exchanged by induction apparatus for currents of very low pressure and great volume. The latter method is the one adopted ill almost all of the apparatus constructed for practical use in electric welding. It enables the dynamo. which is usually made to furnish alternating currents of about 300 volts pressure, to be placed where it is convenient to drive it by power, while the working apparatus or welding transformer may be elsewhere located, two wires of moderate section being used to convey the current front the dynamo to the transformer. The dynamo may be of such size as to be able to supply current at the same time to several welding transformers, or welders, as they arc called, and located in different parts of a manufacturing establishment.
The general character of the apparatus may readily be seen by an examination of Fig. 1, which represents the second machine made, and which machine has become historic.
'Phe primary, P, Fig. 1, is a large, open ring, and is composed of many turns of insulated copper wire. The secondary, S N, is simply a single heavy bar of copper bent to make only one turn outside the primary coil: its ends are turned outward, and provided with powerful screw clamps, C C, for holding the pieces, B B', in place and in abutment. The form of thesecondary is somewhat like a .Jew's-harp, with the clamps on the ends of the parallel portion. The bar, S. is I binned at A and broad ened there so as to give a certain flexibility. A powerful screw and spring at J forces the clamps together when the apparatus is used. Over both primary and secondary a heavy sheathing of iron wire is wound, forming vir tually an endless magnetic circuit of iron around them. The iron wire is wound npon a casing which encloses the two coils, P and S, and prevents the iron wire from interfering with the free movement of the parts of the bar„.;. and the clamps, C C. The resistance of the secondary bar is about .00003 ohm. Vigorous alternating currents, of comparatively high potential, passing in the primary circuit, 1', generate in the bar, S. when its circuit is closed by pieces, B B. to be welded, a low elect ro-tnotivc force acting over a circuit of very low resistance, and giving rise therein to currents of enormous volume. To prepare the pieces for the operation of welding by electric means, all that is necessary to be done is to clean those parts of the pieces which enter the clamps by filing or emery, and to see that the ends or surfaces to be welded are clean enough to effect a contact when pressed together after placing in the clamps. The shape of the abutted ends matters little, as a joint will be formed even when the ends are irregular, but it is better to have the surfaces either flat or with the edge chamfered a little, or with one or both surfaces made somewhat convex, in order that the joint may begin in the middle of the abutted section. The pieces are placed in the chumps. with the ends to be joined projecting therefrom a small amount, and a moderate pressure tending to hold them in abutment, is applied. Sometimes
at this stage a flux, as borax, is added, after which the current is put on. Beating of the abutted ends at once begins and proceeds with a rapidity depending on the current flow, and the size and nature of the pieces treated, reaching the welding heat or temperature of million for the metal, or even reaching the point of actual fusion. With great energy of current. joints on iron bars of over 3 in. diameter hare been made in less than three seconds after applying the current, and with small wires the action is almost instantaneous. The scale on which the apparatus is constructed depends, of course, on the character and dimensions of the pieces to be treated or worked. Wires of 4. of an inch in diameter up to bars of several inches in diameter may be welded by suitable sizes of welders. The current strength required in such ease depends on the nature of the metal or alloy as regards fusibility, specific heat, resistance, etc. Easily fused metals, like tin or lead, require less current, because the tem of welding is just short of their fusing points, which are, of course, comparatively ow, while their higher specific resistance to the flow of current, as compared with iron or copper, still further lessens the current required to produee the beat in any given section.
The metals silver and copper. which, in their pure state, are the most perfect electrical conductors known, and which at the same time possess a very high heat-conducting power, require for electric welding currents of relatively much greater amount than do iron, platinum, gold, etc. The conductivity for heat tends to cause a rapid transfer of heat from the joint to the clamps during the operation, which loss of power is largely kept down by making the weld in as short a time as possible. The conducted heat, as well as the heat developed by the current in passing from the clamps or current-applying contacts to the work-piece to be welded. tends to raise the temperature of the clamps or contacts, and so in a measure lessen their efficiency for conveying current, and also to injure them by oxidation. These parts of the apparatus being usually of copper, or alloys rich in copper, are, however, in the actual machines kept cool by a circulation of water, and are in this way preserved from deterioration. To this end they are on the larger machines constructed with water passages which are connected to a water supply. In such machines also it is not unusual for hydraulic force or hydraulic cylinders to be provided for forcing the pieces together during welding, and sometimes also for holding the pieces in the clamps. The ease and quickness of action and perfection of control by simple valves renders hydraulic pressure peculiarly well adapted to impart the movement necessary for clamping, unclamping, pressing together in welding, etc., dnring the operation. In like manner the current applied in doing the work is controllable and regulable by simple switches and regulating appliances, so thatihe heating effects can be nicely adjusted to the size and character of the work. To such an i extent is this true that electric welding machines are constructed which are automatic in character, or in which the conditions for successful welding. including the amount of current, pressure to be exerted, and point of cutting off of current, having been once determined for the sizes of work for which the machine is adapted, such work may be indefinitely repeated, since the placing of the pieces in the clamps is in accordance with set gauges on the welder. The application of current is after a certain interval followed by its automatic cutting off, and the pressure exerted to close or effect the weld is automatically applied by springs or by a definite hydraulic force. The tendency in the development of the welding apparatus is to have its act-ion automatic even in the case of large work, at least in so far as the application of a certain amount of pressure in forcing the pieces together is concerned. In some cases the automatic character of the apparatus is more complete. This is the case in machines which automatically feed the pieces into place in the clamps, clamp or hold them, automati cally apply the proper pressure and current, allow the proper amount of Yield to take place in forming the weld. and automatically cut off the current, followed by the automatic release of the clamps and discharge of the pieces.