R. G. Griswold.

In order that the strength of a current may be varied at will within certain limits, it is necessary to insert in series with the source of supply a variable resistance. The instrument performing this function is called a rheostat, and, as described herein, has a total resistance of about 47 ohms through 150 steps of .313 ohm each. It has a capacity of 6 amperes and should not be forced to carry more. Fig. 1 shows the instrument complete.

The spool upon which the wire is wound is made of three circular pieces of hardwood, 6 in. in diameter by 1/2 in. thick; separated by two pieces 5 in. in diameter and £ of an inch thick. These five pieces are glued together, heated thoroughly in an oven to expel all moisture and given three coats of shellac. Prepare three 1/2 in. strips of thin asbestos paper equal in length to the circumference of the 6-in. circles, or about 18 7/8| in. Lay off a centre line on two of them 1/4 in. from the edge and running the entire length, and divide into 1/8| in. divisions. Wind these strips around the edges of the two outside disks and the unmarked one about the middle disk, cementing them in place with a light coat of shellac. The divisions of one strip should come exactly between those on the other, as shown by the dimensions in Fig. 2. Give the strips a coat of shellac.

The pins upon which the resistance coil is wound are 3/4-in. wire brads, driven in the two outside disks at the eighth-in. divisions, allowing them to protrude about 1/8 in. The coil is made of about 60 feet of No. 24 B. & S. soft-drawn bare German silver resistance wire, wound from one pin to the next opposite consecutive one. To facilitate the work, fasten the spool in a vise and pass the loop made atone end of the wire by soldering (b Fig 2) over one brad. Then, while holding the wire very taut, pass it from one brad to the other. If the turns about the brads do not lie close to them, they may be made to do so by closing them with a pair of very sharp-nose pliers, easily made by grinding the ordinary kind to an edge at the end. When the last turn has been wound on, it is fastened by soldering to a small brass clip secured to the bottom of disk c. Should an odd spacing of the brads bring this end to the top, fasten it there, bringing the end down through a hole in the spool to the under side where it can be brought out in a small groove. It cannot be brought from the top to the binding screw on account of the brush arm.

The protruding ends of the brads should be sent back over the wire loops, which will serve to tighten them, and the heads cut off with a pair of cutting pliers. Then carefully file the ends down until they are just above the wire and will not interfere with the arm. Wind two or three layers of shellacked thin paper strips, 1/4 in. wide, over the wires just beside the brads, and on these strips wind very tightly several turns of No. 26 B. & S. spring brass wire for binding the wire loops in place in case they should become so heated as to expand enough to slip over the brads. Solder this binding wire at several places to keep the turns together and from slipping over on to the coil. Adjust the various wires where they pass the middle strip of asbestos paper until the spaces between them are about even, and then give them a thorough coat of shellac, both at the middle strip and on the ends where they turn about the brads, where it should be well soaked in. Place in an oven and bake until perfectly dry, when a second coat should be given and baked. A piece of felt'or hard cloth moistened with a little alcohol will remove the shellac from the surface of the wires where the brush makes contact, while that baked on the middle disk will hold them firmly in place where it passes over them.

The brush arm bears on a ring of 1/8 in. hard brass wire, fastened to the spool by means of small clips d, Fig. 1, soldered to it and set in recesses in the top so that the ring has a solid bearing. One of the connecting wires for the binding posts is soldered to this ring at d, and carried down through a hole to a groove in the bottom and thence to the binding screw. The top of this ring should be slightly flattened and polished perfectly smooth.

Fig. 2.

Fig. 3.

The brush arm is made from a piece of 3/32 in. sheet brass, carrying at its outer end a contact brush made of sixteen pieces of No. 32 spring brass or hard-drawn copper wire, about 1 3/2 in. long and bent as at e, Fig. 3, and soldered to the arm. Arrange these pieces so that they all lie in the correct position between two pieces of thick pasteboard, when they may be grasped with the pliers and held in position while being soldered to the arm. Bend the wires as shown so that they bear firmly on the coil wires, but not hard enough to displace them when moving. The bearing surface should be polished very smooth so that the brush will slide easily.

The construction of the hand wheel and stem is plainly shown in Fig. 1. The box should be made of well seasoned hardwood and provided with the ventilating holes shown. The feet on the bottom permit of plenty of room for air to reach the bottom holes, and as the passage of the current heats the coil a circulation of air is established. After centering the spool on the bottom secure it in place with six screws, so adjusting the spool that the grain of the bottom disk and that of the bottom of the box cross at right angles to prevent warping. The top is made in two pieces so that the interior is accessible without removing the hand wheel. Bend the two sections of the brush arm s and s' slightly, to form a spring bearing on the wire ring, which will insure a firm contact and prevent binding at any part of rotation. Adjust the pressure of the arm on the ring by means of the screw f until it turns smoothly, connect the coil to one binding post and the bearing ring to the other, and the coil is ready for service. The box may be finished in any desirable manner.

This rheostat is not designed to carry a heavy current for any length of time, but simply to afford a means of quickly varying a resistance for experimental purposes, such as in the calibration of galvanometers where the adjustments of resistance are required in small steps.