For light currents in experimental work, where batteries are employed, obtain a pound or two of bare German-silver wire, from Nos. 24 to 30, and wind the strands on a round piece of stick attached to a winder (see Magnets and Induction-Coils, chapter iv.). Make several of these coils, two or three inches long, with the wire wound closely and evenly. When pulled apart the coils will appear as shown in Fig. 2 A, and will resemble a spiral spring. This can be made fast over a porcelain knob and the ends caught down, as shown at B in Fig. 2, or it may be drawn over a round stick, a porcelain tube, or a lug made of plaster of Paris and dextrine (three parts of the former to one of the latter), as shown at C in Fig. 2, and the ends securely bound with a strand or two of wire, twisted tight to keep the ends from slipping. The lugs may be made in a mold, using as a pattern a piece of broom-handle--shellacked and oiled to prevent the plaster from adhering to it. Obtain a small square and deep box, and drop some of the wet mixture down in the bottom; on this place the broomstick, small end down (it should be slightly tapered), and around it pour in the wet plaster mixture. While it is setting, turn the stick with the thumb and fingers, so as to shape the hole perfectly then draw it out, and a true mold will be the result. When dry enough, pour some shellac down into the mold and revolve it, so that the shellac will be evenly distributed, and let it harden for a day. Then saw off the end of the mold, so that it will be open at both ends. 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. [Illustration: _=Fig. 3=_]