Page:Popular Science Monthly Volume 90.djvu/644

This page needs to be proofread.


��Popular Science Monthly

��A Handy Method for Voltage Testing of Storage Batteries

TESTING a battery of storage cells to determine whether or not they need recharging is a very difficult matter when the batteries are put in an out-of-the-way place not easy to reach. But in just such a case as this, the method described herein is particularly useful. A specially constructed

���1 Z J

Wiring diagram for voltage testing each cell, and the cross-section of the switch connection

change-over switch is mounted on the same panel with the voltmeter. The switch con- tains two contact arms capable of being rotated in as many positions as there are batteries to be tested. The voltmeter is connected with the pivot-end of the switch and the individual cells are them- selves connected with the contact-points surrounding this pivot. The contact arms or blades A and B are made of strips of spring brass or copper, and two small blocks of fiber, H and K, are fastened to them by machine screws to keep them apart. The rectangular arrangement so formed revolves about the larger machine screw D, which passes through a hole drilled in the center of the block K through a suitable washer E, finally into another hole in the panel M. The two nuts, P, hold the screw D in position and at the same time provide a means for attaching an electrical connection. The blade B is electrically connected with the screw D by means of the connecting strip F, as shown. The blade A makes another out- side connection through the circular brass ring 5 and the brush G which touches it no matter in what position the blade hap-

��pens to be. The connection with the ring 5 and the connection with the screw D lead to the respective terminals of an ordi- nary I- to 3- volt voltmeter.

The contacts surrounding the pivot of the switch are made of machine screws and are spaced apart from each other a distance equal to the distance between the two blades A and B. This switch requires the number of contacts to be only one more than the number of cells that are connected in series. The connections are made with the contact-screws in the rear of the panel, and adjacent contacts are connected with adjacent posts on the cells, as shown in the diagram of connections. From this diagram it is evident that when the blades are in the position i, the voltmeter will be con- nected with cell number i. In the position 2 shown in the diagram, the voltmeter will be connected across cell number 2; and so on. Thus, in testing each cell, it is only necessary to turn the switch.

��How to Handle Hard Rubber to Prevent Its Breaking

HARD rubber is one of the materials most frequently used by the electric experimenter; first because of its excellent insulating qualities, and second, because it is easier to handle than porcelain or similar hard materials. Nevertheless, a great deal of the material is often wasted before the desired results are accomplished, because care is not taken in the working of it.

Hard rubber can be cut without chipping, with a hacksaw or a fine-tooth saw if the cut is made more than half way through on one side, then started on the other side until the first cut is met. The pressure on the saw should be very light or the rubber will crack. Rubber can be drilled with ordinary twist drills in a drill-press; but the stock should be laid on a perfectly flat surface and a very light pressure applied on the drill. When finishing a base it is often desirable to have the edges rounded. This can be done with fine sandpaper. To obtain a dull finish on the surface it should first be sandpapered and then washed. A brighter finish can be obtained by mixing a little emery with oil and polishing the surface.

When working hard rubber in a drill- press it is best to have it fastened to a solid support so that the drill will not bend, or the material lift up with the turn of the tool. — Alexander V. Bollerer.

�� �