# Page:Popular Science Monthly Volume 91.djvu/332

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��merit and rheostat in the circuit. The two brass contact-arms on the switch-handle are insulated from each other and are of the edgewise-contact type. They should be made of thin spring-brass and split at the end as shown, so that each blade will make perfect contact with two points at a time. The switch-arms are fastened to the handle by laying them on the back of the insulating

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��A polarity changer for reversing a cur- rent taken from a lighting battery circuit

handle in their proper position, placing over them a fiber-washer and drawing it up tight against the blades by means of a nut screwed on to the bolt through the handle. Adjust the knob so the ends of the blades strike the contacts in the center. Then adjust the blades so that when one arm is in contact with No. 2 and 1, the other is in contact with No. 3 and 4; and when one connects No. 2 and 3 the other connects 4 and 5. Care should be taken to see that all contacts are the same height and all equally spaced ; for then it will be easy to get each arm to touch two contacts. A good holder for the handle is made of an old binding post which has a wide base. Force the post into a hole in the base with the base projecting, in order to keep it from going clear through. The shaft on the handle is then screwed into this until the contacts are made certain by the pressure of the spring. Be sure that a good contact is made, otherwise the instrument will be inefficient. — Frank Sahlman.

��A Simple and Dependable Multiplication Method

AN interesting and simple method of multiplication is performed as follows: Suppose, for example, that it is desired to multiply 145 by 39. Write 39 in one column, 145 in a second. Divide 39 by 2,

��neglecting the remainder, and multiply 145 by 2; write the first result in the first column, the second result in the second. Continue the process of dividing the num- ber in the first column by 2 and multiplying the number in the second column by 2 (always neglecting the remainder if one occurs in the division) until the number in the first column is 1. Then strike out all of the numbers in the second column that are opposite even numbers in the first column and add the numbers remaining in the second column. The result will be the required product. The work for this problem is shown :

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��The method depends on the fact that any number may be expressed as the sum of powers of 2 (including 2° = 1). The number 39, for instance, is equal to 2° + 2 1 + 2 2 + 2 5 , that is, 1 + 2 + 4 4- 32. The numbers remaining in the second column are 1 x 145, 2 x 145, 4 x 145, and 32 x 145, so that their sum is equal to 39 x 145. — Paul R. Rider, Ph.D., In- structor in Mathematics, Yale University.

��An Insulation for Secondary Terminals on Transformers

THE insulation of secondary terminals on home-made transformers is often very poor, resulting in leakage and low- ered efficiency. The hard rubber shells from telephone re- ceivers can be used in such cases with excellent results. The shells should be mounted as shown in the illustration. A long brass rod threaded on both ends is run com- pletely through the device and clamps ™ cotL " it firmlv in oosi- Method of mounting the it nrmiy in posi sheUs for insulation

tion. Connections

are easily made with both ends of this rod.

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