468 



Popular Science Monthly 



Scale for Inductive Coupler for 

 Making Log-Book 



PROCURE a wooden ruler and cut it 

 the proper size to fit between the 

 front end of the primary and the support 

 for the secondary rods. Fasten it so 

 that the coupler secondary will clear it, 

 and attach a small pointer to the sec- 

 ondary coil end. This will serve as a 

 guide when tuning for distant stations 



RULER 



An ordinary ruler fastened between pri- 

 mary coil end and support for a scale 



and is also very handy in making out a 

 log-book. Once you have used a scale 

 to mark the coupling of your receiver 

 you will always want one. 



Select an ordinary ruler such as found 

 in school outfits, which has graduations 

 no less than 8 to the inch — 16 will be 

 much better. — Samuel Huff. 



Heated Wire Is Telephone 

 Transmitter 



IT is an interesting fact not generally 

 known that an exceedingly fine wire, 

 red hot, will, under suitable conditions, 

 serve as a telephone transmitter. Sound 

 waves striking the wire vary its resist- 

 ance to such an extent that the variations 

 are plainly noticeable in a sensitive tele- 

 phone receiver. 



A Russian inventor has recently 

 brought out an improved type of "ther- 

 mo- telephone," as it is called, in which 



Wire embedded in block of suitable insu- 

 lating material and used for a transmitter 



the heated wire is almost entirely en- 

 closed in a protective body. His inven- 

 tion includes a number of ways of em- 



bedding the wire. In one case, it is cast 

 in a block of suitable insulating material. 

 In others, it is merely held tightly be- 

 tween the surfaces of close-fitting hinged 

 plates. 



In use, the ends of the heated wire — 

 which is usually Wollaston or very fine 

 platinum — are connected through a bat- 

 tery to the receiver. Sound waves strik- 

 ing the block increase or decrease the 

 resistance of the wire, allowing more or 

 less current to flow through, with a con- 

 sequent variation of the sound being 

 emitted by the receiver. 



Device for Testing Spark-Plugs 

 Without Removing Them 



DESIGNED to enable spark-plugs to 

 be tested without unscrewing them 

 from the cylinder, the simple device 

 shown in the accompanying views con- 

 sists of a T-shaped handle of non- 



i^- 



An insulated handle for making spark gap 

 on an engine plug without unscrewing it 



conducting fiber to which are attached 

 two pieces of zinc wire of unequal length. 

 The wires are bent as shown and are 

 held in place on the fiber handle by 

 means of two binding posts, the ends of 

 the wires between the posts being about 

 the thickness of a dime apart. 



In operation, the main spark-plug 

 wire is kept in place to furnish current 

 to the plug and the short wire of the de- 

 vice placed on the top of the plug 

 electrode and to the long arm on the 

 cylinder. Then as the motor is turned 

 over, a spark will jump the gap between 

 the ends of the zinc wires if the plug 

 is in good condition. If short-circuited, 

 no spark can be secured. If the por- 

 celain is defective the spark will be 

 weak and irregular. 



