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Page:Popular Science Monthly Volume 89.djvu/646

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��Popular Science Monthly

��energy from primary to secondary is secured with couplings as iiigh as 40 per cent; this close hnkage rccjuircs an ap- parent detuning for the best results. It is preferred to have the resonant frequency of the aerial circuit somewhat higher than that of the highly-damped jjrimary.

Several other ways of relating the two circuits and the microphone are shown in the patent. In one of these the telephone transmitter is placed in an intermediate coupling-loop which con- tains no condenser and is therefore a periodic. Other minor variations are possible. The f)ower leads may be connected across the condenser P instead of across the spark-gap, or .several spark-gaps may be used in series. In operation the voltage and gap-length are adjusted so that sparks are produced at a very rapid rate; each spark creates a rapidly damped train of oscillations in the primary, and the energy of these is transferred to the .secondary, where the result is a series of feebly damped and practically constant-amplitude high-fre- quency currents. The strength of these antenna currents is altered by the resistance changes in the microphone, and speech-waves are thus radiated.

��Tracing Initials on Tools with Electricity

TCJOLS may be marked very easily with electricity, wiiich is easier than marking with acid. Screw the plug of an old cord into a socket, and wrap wire D around point A. Connect anotiier wire with B, which is connected with the tool to be marked. The other


��A — n



��"^ CARBON Connections to a rheostat and carbon for writing letters on steel

Strand of wire C is connected with the graphite from a pencil or a i>icce of carbon frdin a dry batler\ . If carbon is u.sed, care should be taken in sharpening it to a fine point, since it is extremely brittle. After insulating the wiioK- ar-

��rangement, the initials may be traced. The carbon makes deep, sharply-defined outlines. Graphite lines are shallow and

rough. H. WORTHMANN.

��A Switch Operated by Pressure on a Footboard

THIS type of switch will be found useful in a wireless station, espe- cially for a test buzi^er. It is operated

���The switch is fastened to the underside of a table or shelf

by pressing on the footboard.

A common S.P.S.T. switch has a bent spring of stiff brass fastened to the base and so adjusted as normally to keep the switch l)lade away from the jaw.

The switch is fastened to the under- side of the table or shelf. A stout cord is tied to the handle and jiasseii over a l)ulley, and the other end is fastened to a hingeil piece of board, as shown in the drawing. — John B. Rakoski.

��Differences in Time Between New York and Foreign Points

EXPi:RIMb:NTi:RS are sometimes confused by the difference in time used at various foreign points. France and l-^nglaiul time li\e hours ahead of New York, ('icrm.iny's time is six hours ahead. San I-'r.mrisco is three hours later than New York, and Hono- lulu time is 5>^ hours slower .New York.

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