This style of resistance equipment is the least expensive to make, and will give excellent satisfaction to the boy who is using light currents for induction-coils, lamps, galvanometers, and testing in general. The simplest form of liquid resistance is made by using a glass bottle with the upper part cut away. The cutting may be done with a steel-wheel glass-cutter. The bottle should then be tapped on the cut line until the top part falls away. Go over the sharp edges with an old file to chafe the edge and round it; then solder a tin, copper, or brass disk to a piece of well-insulated wire and drop it down in the bottom of the receptacle, as shown at Fig. 17. Cut a smaller disk of metal, or use a brass button, and suspend it on a copper wire which passes through a small hole in a piece of wood at the top of the jar. Notches should be cut at the under side of this wood cross-piece so that it will fit on top of the jar and stay in place. The jar is to be nearly filled with water, having a teaspoonful of sulphate of copper dissolved in it. This will turn the water a bluish color and make it a slightly better conductor, particularly when the button is lowered close to the round disk. If a high resistance is desired the copper may be omitted leaving the water in its pure state. The wires leading in and out of the jar should be connected between the apparatus and the battery so that the proper amperage can be had by raising or lowering the button. A series of these liquid resistance-jars may be made of glass tubes an inch in diameter and twelve inches long. One end of them may be stopped with a cement made of plaster of Paris six parts, ground silex or fine white sand two parts, and dextrine two parts. Mix the ingredients together when dry, taking care to break all small lumps in the dextrine; then add water until it is of a thick consistency like soft putty. Solder the ends of some copper wires to disks of copper or brass and set them on the middle of bone-buttons; these in turn are to be imbedded in the mixture after the wire has been passed through a hole in the bottom. Their location can be seen in the bottom of the tubes Fig. 18, and Fig. 19 A is an enlarged figure drawing of the plate, button, and wire. The wires are brought out under the lower edge of the tubes, and enough of the composition is floated about the bottom and outer edge of the tube to form a base, as shown in the drawing. A base-board is made six inches wide and long enough to accommodate the desired number of tubes. Two pieces of wood one inch wide and three-quarters of an inch thick have hollow notches cut from them at one side, as shown at Fig. 19 B. In these notches the tubes are gripped. Screws are passed through one stick and into the other so as to clamp the wood and tubes securely together. The rear stick is supported on two uprights which are made fast to the rear edge of the base-plate with screws and glue.