632 



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^^ 



A 

 — n 



WIRE TO TOOL 



\ 



"^ 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 u.se 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 tli.in .New 

 York. 



