Popular Science Monthly 



275 



How a Compact Molded Con- 

 denser Is Built 



U. S. PATENT No. 1,174,600, issued 

 to W. J. Murdock, shows the con- 

 struction of the molded condensers 

 which have become familiar in wireless 

 telegraph sending stations. By first 

 fixing in position the plates of the con- 

 denser and then casting them scjiidly 

 into a mixture of pulverized mica and 

 resinous gum, the inventor states that he 

 secures a condenser which is compact, 

 strong, efficient and inexpensive. The 

 drawings show how the two sets of 

 plates a, a, a, and b, b, b, are connected 

 to the terminals e and g by means of 

 the strips d and f. The terminal screws 

 pass through conducting strips h, i, 

 which have their adjacent ends beveled 

 and form a protective spark-gap n. 

 The molded dielectric material, c, is 

 uniform throughout and there are no 

 insertions of mica sheets or other sepa- 

 rators. Units of con\'enient capacity 

 and voltage may be made up and com- 

 bined in series or parallel to meet the 

 needs of any particular transmitting 

 outfit. The condenser has found ex- 

 tended application in portable small- 

 powered quenched-gap senders. 



i : \ 



Diagram of a molded condenser for small 

 quenched-gap senders 



T 



Small Radio Stations 



HERE are in the United States 

 three licensed amateur stations using 

 only five watts inpud power in their 

 transmitters. Several others use six, 

 eight and twelve watts. It requires 

 about fifty watts to light an ordinary 

 carbon - filament sixteen - candlepower 

 electric lamp. 



Primary Regulator for the 

 Induction Coil 

 The induction coil is shown by A. 

 B represents the battery, and 5 a five- 

 point switch. M-i, M-2, M-3 and M-i^ 

 are magnets from old electric bells. By 

 switching in one or more of these the 

 I)ower used by the coil can be reduced as 

 desired. — C. S. Pokter. 



— 5 



An arrangement for regulating the power 

 used by an induction coil 



Cardboard Tubes 



A GREAT many amateurs, when mak- 

 ing tuners or loose couplers, are 

 puzzled when they come to the card- 

 board tube problem. They either do 

 not know where to buy the tubes, or 

 cannot make them successfully. How- 

 ever, these tubes can be easily made. 



The first thing to do is to get a round 

 cylinder the desired length and diameter 

 and some thin cardboard or thick 

 paper. Wind the paper or cardboard 

 around the cylinder. After the first 

 la\er has been put on coat the inside of 

 the paper with glue to make the layers 

 stick together. After the paper has 

 been wound on to }s or 3 16 in. thick- 

 ness, put elastic bands around the tube 

 to hold it from becoming loose. After 

 the tube has become thoroughly dry 

 take it off the cylinder and coat it with 

 shellac. This process serves to strengthen 

 it appreciably and makes it ready for 

 use. — Albert Killmeykr. 



