It has been found that the best form of transformer is the one in which the primary and secondary coils are eo-axially placed on nn iron core, so as to oblige all lines of force generated by the former to cut through the latter. If the two windings are placed side by aide on an iron core or on opposite legs, as shown in Fig. 135, there is a tendency to form consequent poles at the point where the two windings meat, due to the leakage of lines of force. The leakage will depend upon the distance between the two legs and upon the length of magnetic circuit. It has been purposely increased and turned to advantage by Prof. Thomson, for obtaining constant current in the secondary while the primary is supplied with constant E.M.F., Fig. 136. When, however, constant E.M.F. in secondary fs required, it is important that this leakage should be decreased to a minimum.
Various forms of secondary may be employed to work consistent with the above principle. A few of these arc shown in Figs 137,138,139, and 140, in which cases the secondary is preferably a solid copper casting. Forms shown in Figs. 140 and 141, however, permit hard rolled copper plates to be used.
For mechanical reasons and convenience, forms Nob. 137 and 138 are mostly used, and actual experience shows practically no leakage. This construction permits of removing the primary without disturbing the secondary in the least. It also gives the least possible self-induction for a given cross section.
It is often found stated that when large quantities of alternating currents have to be conveyed through copper conductors, it is important to have the same laminated or subdivided in smaller conductors to reduce self-induction.
The sehapa of cross section is, however, far more important than the lsmina-
An eiperimentsl compound conductor, consisting of a number of flat copper ribbons in parallel, showed a self-inductiou of three when the ribbons were closely packed, of two when separated into two parts, and one when spread open like a fan.
The explanation for this was given by Prof. Thomson, when he showed self-induction to depend mostly upon the length of the lines of force surrounding a conductor when traversed by intermittent or alternating currents.
Arrangement of Clamps.
The pole which is insulated from the table constitutes the stationary clamp. The uninsulated pole, which is considerably larger, has on its upper side a long V-shaped groove parallel with the axis of the secondary, Fig. 142. A movable copper block, also V-shaped, fits in the bearing, and can be slid in the same forward and backward.
This movable block carries the second clamp. The current necessary for welding has to pass through this sliding contact. The welding of small copper wire, or any other easily fusible metal, is a very difficult thing, if special apparatus is not used. The current required is very large, several hundred amperes for wire not larger than No. 17 A.G.
The metal when it reaches welding heat readily melts away, and has to be followed by the movable clamp, so as to prevent the breaking of the circuit. This latter action, when it occurs, is so violent that a special device is necessary to prevent injury to the coil in case of such a rupture. The movable clamp, in order to follow the softening of the metal, has, therefore, to have as little friction as possible, and yet has to make a good contact to carry the heavy currents. The clamp is furthermore required to move in true guides so as to abut the small conductors with their axes in line. The heating necessarily brought about when rapid and continuous welding is done roust not interfere with the bearing, and cause the carriage to stick through expansion.
All these conditions have been complied with in using the V-bearings, the carriage being held down in contact by means of a heavy spring, and number of copper rollers being interposed between the carriage and the bearing, Fig. 142. The copper rollers are simply short pieces of 1/4 in. hard-rolled copper wire rounded at the ends. There are 16 altogether, 8 towards the front and 8 towards the back of carriage, equally distributed on both sides of the bearing.
A stationary rod at apex of bearing prevents the interference of the two rows of rollers. Between the two sets of rollers in front and in back a bolt passes applying the spring pressure, forcing the carriage into contact with the bearing. A pressure of 401b. may be employed, and yet the carriage will move freely. These 16 rollers have to transmit a current of approximately 3000 amperes. They absorb about 20 per cent, of the total energy, which loss is, however, fully balanced by she reliability and simplicity of the device. This loss is, moreover, only on the maximum sizes, and becomes insignificant on small work.
An adjustable coiled spring pulls the sliding clamp toward the stationary one. In front and pivoted on a lever is the distance gauge, which may be inserted between the two clamps. This gauge carries on a central disc a number of steel pins of varying lengths, but equal projection on either side of disc. These projections, if. inserted between the two clamps, give the necessary distance required for a given size wire, the disc against which the wires are abutted ensuring equal projection of both ends to be joined. On the back of the apparatus a switch is located in the primary circuit, which is normally held open by a spring. By moving the handle to the right the primary will be closed, and the switch locked by a little catch underneath the table. A pin fastened to movable clamp will lift the catch and release the switch, thus opening the circuit.
An intermediate lever between catch and pin operated by a sliding rule, which serves also as an index, permits of varying the point of cut-off, which has to be in a certain relation to the distance between clamps. The position of cut-off, tension of spring, distance between clamps corresponding to each other, are marked with the same figure, which is also inscribed on a wire gauge fastened to the welder. By inserting a wire into the gauge the number read will give the necessary adjustments at once.