A Simple Pocket Indicator for Finding the Negative Wire in Battery Cable Lines
To use, connect the terminals to the battery lines, and the end of the wire in the solution giving off bubbles is the negative wire.—Contributed by H. S. Parker, Brooklyn, N. Y.
Small Steam-Engine Cylinders Made from Seamless Brass Tubing
In making a small steam engine it was desired to use seamless brass tubing for the cylinders. To have them exactly alike a piece of tubing of the right size and of sufficient length for both cylinders was fitted on a wood mandrel, A, and the ends trued up in a lathe. As these cylinders were to fit into holes bored in a steel bedplate, it was necessary to have a flange at one end. A groove was turned in the tubing B in the center, and as a final operation a parting tool was used on the line CD. This resulted in a pair of cylinders flanged to fit the bedplate.—Contributed by Harry F. Lowe, Washington, D. C.
A Wood Mandrel Used to Face the Ends and Turn Flanges on Tubing for Cylinders
A Pocket Direct-Current Voltmeter
The assembled drawings of a very simple voltmeter are shown in Fig. 1, and its operation is as follows: The moving portion consists of a pointer, or needle, A; a small permanent magnet, or armature, B, and a counterweight, C, mounted upon a small steel shaft, D. The ends of this steel shaft are pointed and rest in bearings provided in the U-shaped piece of brass E, which is rigidly fastened to the fiber base F, by means of two screws. The permanent magnet B, carried on the shaft D, is at all times under the magnetic influence of the permanent horseshoe magnet, G, which is fastened, by means of thin brass straps, H H, and small screws, to the base F, so that the ends of the armature B are directly above the poles of the horseshoe magnet. The armature B will assume the position shown in the sketch when it is acted upon by the permanent magnet G alone and the moving system is perfectly balanced. A solenoid, J, is mounted in the position shown. When there is a current in its winding its soft-iron core will become magnetized and the magnetic pole produced at the lower end will produce a magnetic force upon the armature B, with the result that the armature will be rotated either in a clockwise or counter-clockwise direction, depending upon its polarity and the polarity of the end of the core adjacent to it. Thus, if the left end of the armature has north polarity, the right end south polarity, and the lower end of the core is magnetized to a south polarity the armature will be rotated clockwise, for the left end, or north pole, will be attracted by the lower end of the iron core, which is a south pole, and the right end will be repelled. This is in accordance with one of the fundamental laws of magnetism which states that magnetic poles of unlike polarity attract each other and those of like polarity repel each other. The amount the armature B is rotated will depend upon the relative effects of the pole of the solenoid and the permanent magnet G. The strength of the pole of the solenoid will depend upon the current in its winding and the number of times the current passes around the core, or the number of turns in the winding. In other words, the strength of the pole of the solenoid varies as the product of the current and the number of turns, which is called the ampere-turns. The same magnetic effect can be produced by a large current passing through a few turns or a small current passing through a relatively large number of turns. This simple relation of current and turns gives a means of adjusting the current capacity of the instrument so that a full-scale deflection of the needle will correspond to any desired maximum current. The instrument may be used as either a voltmeter or as an ammeter, and its operation will be identical in each case. The resistance of the voltmeter, however, will be many times the resistance of the ammeter, as it will be connected directly across the line, while the ammeter will always be in series in the circuit in which it is desired to measure the current. The following description and suggestion as to how to proceed in the construction of this instrument may be useful to those who undertake to build one. All the minor details and some of the dimensions will be omitted in the description, but these can be easily supplied.