The materials necessary are: One 5-point wood-base switch, 4 jars, some sheet copper or brass for plates, about 5 ft. of rubber-covered wire, and some No. 18 gauge wire for the wiring.
Wiring Plan for Water Rheostat
The size of the jars depends on the voltage. If you are going to use a current of low tension, as from batteries, the jars need not be very large, but if you intend to use the electric light current of 110 voltage it will be necessary to use large jars or wooden boxes made watertight, which will hold about 6 or 7 gal. Each jar to be filled with 20 parts water to 1 part sulphuric acid. Jars are set in a row in some convenient place out of the way.
Next cut out eight copper or brass disks, two for each jar. Their size also depends on the voltage. The disks that are placed in the lower part of the jars are connected with a rubber covered wire extending a little above the top of the jar.
To wire the apparatus, refer to the sketch and you will see that jar No. 1 is connected to point No. 1 on switch; No. 2, on No. 2, and so on until all is complete and we have one remaining point on switch. Above the jars place a wire to suspend the other or top disks in the solution. This wire is also connected to one terminal on the motor and to remaining point on switch. The arm of the switch is connected to one terminal of battery, or source of current, and the other terminal connected direct to remaining terminal of motor.
Put arm of switch on point No. 1 and lower one of the top disks in jar No. 1 and make contact with wire above jars. The current then will flow through the motor. The speed for each point can be determined by lowering top disks in jars. The top disk in jar No. 2 is lower down than in No. 1 and so on for No. 3 and No. 4. The connection between point No. 5 on switch, direct to wire across jars, gives full current and full speed.
A water rheostat may be made by fitting a brass tube with a cork, through which a piece of wire is passed. The brass tube may be an old bicycle hand pump, A (see sketch), filled with water. Pushing the wire, B, down into the water increases the surface in contact, and thus decreases the resistance. An apparatus of this kind is suitable for regulating the current from an induction coil, when the coil is not provided with a regulator, and by using a piece of pipe instead of the tube, it can be used to regulate the speed of a motor.
When the pipe is used, a piece of brass or copper rod should be substituted for the wire, in order to increase the surface. Adding salt to the water will decrease the resistance, and, when used with a motor, will give a greater speed. --Contributed by John Koehler, Ridgewood, N. J.
Diagram of One-Wire Line Water Rheostat