316 



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 



Popular Science Monthly 



Fiber 



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 : 



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. 



