In order to make the lugs, pour in the plaster mixture, taking care to oil the mold before each pouring, so that the lug can be drawn out when the mixture has set. If it sticks, tap the small end gently to start it. For coils where there is little or no heat, ordinary pieces of broom-handle, or round sticks having a coat or two of shellac, will answer very well; but where the current heats the core, it must be of some material that will not char.
Another method of making resistance-coils is to measure off a length of wire; then double it, and with a small staple attach the loop end at one end of the (wooden) core. Pay out the two strands of wire an equal distance apart with the thumb and fingers, and with the other hand twist the core. At the other end of the spool catch the loose ends of the wire under small staples, taking great care not to let the staples touch or even be driven close together. This arrangement is shown at D in [Fig. 2], and for a field resistance-board any number of these coils may be made.
In [Fig. 3] the mode of connecting coils is shown. The dots represent contact-points to which the movable arm can be shifted. The wires at the bottom of coil, Nos. 1 and 2, are connected together, while those at the top of No. 2 and 3 are joined, and so on to the end. The leading-in current is connected at pole H and so on to J, while the leading-out wire is made fast to pole I. The switch-arm is moved on the first dot, or contact-point, and the current passes up wire A, down coil No. 1, up coil No. 2, down No. 3, up No. 4, and so on to No. 6, and down wire G and out at I. Supposing that this offers too much resistance, the switch-arm is moved up one point. This cuts out coil No. 1, as the current passes up wire B, through coil No. 2, down No. 3, and so on, and out through G and pole I. Another move of the switch and coil No. 2 is cut out, the current passing up wire C, down coil No. 3, up No. 4, and so on, and out at I. Each move of the switch cuts out one coil, lessening the resistance; but when moved to the last contact-point the current flows without resistance—in at H, through the switch-arm, and out at I.
The plan of arranging the coils suggested at [Fig. 2] B is shown in [Fig. 4], where four of the coils are arranged in series over porcelain knobs, and the lower ends made fast to the base-board with small staples. Small pieces of brass are used for the switch contact-plates; those are provided with one plain and one countersunk hole for a flat and round headed screw.
The screw-heads are arranged in a semicircular fashion, so that the switch-arm, attached at one end to the screw J, will touch each plate as it is moved forward or backward.
TWO SIMPLE FORMS OF RHEOSTATS
The current passing in at binding-post A travels to J and B, the latter being the resting-plate for the switch-arm. A move of the arm to C sends the current up over the first coil and down; then over the second, third, and fourth coils, and out at G; through plate H (which is the rest at the right side), and out at I.