An Easily Constructed High 

 Tension Insulator 



Popular Science Monthly 959 ' 



Finding the Combined Value in \ 

 Parallel Resistances 



THE drawing below shows a high 

 tension insulator that is easily made, 

 has good strength and insulation, and is 

 businesslike in appearance. It can be 

 made for about five cents. Two cleats are 

 used for average 

 power in ama- 

 teur stations bolt& 



NUT S 



CLEATS 



METAL STRIP 



An easily constructed 

 insulator for use on 

 high tension wires 



while three or more can be used for others 

 of greater potential. The cleats are joined 

 with y%-m. iron or brass strips fastened 

 with small bolts as shown in the sketch. 

 For good appearance all the metal parts 

 should be enameled black, . while the 

 porcelain can be covered with thick 

 shellac, giving it the appearance of brown 

 glaze. — John B. Rakoski. 



Heating Hard Sheet Rubber to 

 Facilitate Cutting Disks 



THE amateur experimenter and worker 

 on electrical apparatus at some time 

 or other desires to cut hard sheet rubber 

 into round disks for making Wimshurst 

 machines or other devices. Hard rubber, 

 as received from the stock house, is very 

 difficult to cut, unless you have all the 

 facilities for doing such work. 



The writer has found a very easy way 

 to cut hard sheet rubber up to 3/16 in. 

 thick. The method is as follows: Mark 

 on the rubber with a scriber or other sharp 

 instrument an outline of the piece to be cut. 

 Then plunge the sheet into hot water; 

 take it out and cut on the outline with a 

 pair of scissors. The rubber will become 

 soft like leather and cut easily. As it 

 becomes cooler, it will cut harder. If 

 any more cutting is to be done, plunge 

 it into hot water again and continue until 

 the cutting is done. Then put a plate of 

 glass on a newspaper on a table, dip the 

 rubber in the hot water again, place on 

 the glass; then put another sheet of glass 

 over the rubber with a weight on it. 

 When it is cooling the piece will straighten 

 out. — W. S. Standiford. 



WHERE more than two resistances ] 

 are connected in parallel it usually 

 results in rather complicated calculations 

 in order to determine the combined re- 

 sistance value. It has been found that 

 an easy way to determine the equivalent 

 of resistances in parallel is by the use of a 

 simple diagram, as shown. • 



As an illustration, the four resistance 

 values, 225, 450, 900 and 1800 ohms 

 were taken. The procedure is as follows: 

 Decide on some convenient scale to be used. 

 In this instance, each 1/10 in. equals 

 50 ohms. Lay off a perpendicular line 

 X- Y to represent any one of the resistances, 

 the one of the highest value, which in 

 this case is 1800 ohms, probably being 

 the most convenient. Then construct the 

 horizontal line Y-Z of any convenient 

 length- and erect the perpendicular A-Z 

 equal in height to the next highest re- 

 sistance. On this line lay off the remaining 

 resistances according to the scale selected. 

 For convenience we 

 xisoo will start with the two 



highest values, although 

 the same results fwould 

 be accomplished by us- 

 ing the 1 800-ohm resis- 

 tance and any one of 

 the other three shown 

 on line A-Z. Draw 



A convenient scale for finding the com- 

 bined value of resistances in parallel 



lines X-Z and A-Y. At their point of 

 intersection, B, erect the perpendicular B-C 

 whose value represents the equivalent of 

 the 900 and 1 800-ohm resistances in 

 parallel. Combine this equivalent with 

 the 450-ohm resistance by drawing line 

 H-C. At the point of intersection D of 

 this line with the line B-Z, which happens 

 to fall on the line X-Z, erect the perpen- 



