Page:Popular Science Monthly Volume 90.djvu/484

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��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


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-


���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.

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