Popular Science Monthly 



305 



the spring C is released by the break- 

 ing of the thread, the weight A swings 

 to the left. By reason of its inertia the 

 weight does not stop at the central nor- 

 mal resting position unless the friction 

 is very large, but "overshoots" and trav- 

 els off to the left side. But its motion 

 to the left does not carry it so far from 

 the center as it was originally. When it 

 again swings back to the right the dis- 

 placement is still less ; the successive par- 

 tial mechanical oscillations to right and 

 left gradually become smaller until the 

 energy originally imparted is used up, 

 when the swinging stops. 



For every complete oscillation of the 

 freely vibrating antenna system a certain 

 definite time is required. This time, 

 which is usually measured in fractions 

 of a second and is called the period of 

 oscillation, depends upon the capacity 

 and inductance of the vibrating system. 

 It is a definite quantity for each amount 

 of capacity and inductance, and, when 

 the resistance is not abnormally high, 

 depends only upon these. If the capac- 

 ity of the circuit is stated in farads and 

 the inductance in henrys, the time of one 

 complete oscillation in seconds may be 

 found by (first) multiplying the capac- 

 ity by the inductance, (second) taking 

 the square root of this product, and 

 (third) multiplying the result by 6.28. 

 Thus if the capacity is 0.002 microfarad 

 (or 0.000000002 farad) and the induct- 

 ance 3.2 millihenrys (or 0.0032 henry), 

 the product is 0.0000000000064, its 

 square root is 0.00000253, and the period 

 (multiplying by 6.28) is about 0.0000161 

 of a second. The frequency is obviously 

 the reciprocal of this, or 62,000 periods 

 per second, which (as shown last month) 

 corresponds to a wavelength of 4,800 

 meters. 



In the next article some of the effects 

 of changing inductance, capacity and re- 

 sistance in both open and closed circuits 

 will be discussed. 



Edison's Railroad Wireless 



WIRELESS was used on railroad 

 trains as long ago as 1885, but 

 the system then devised by Edison de- 

 pended upon static induction and not 

 radiated waves. It has been only re- 

 cently that radio telegraphy has 

 proved useful in railroad work. 



A Roof Insulator 



AN insulator for lead-in wires passing 

 over the edge of a house roof may 

 be made by cutting a piece of stiff asbes- 

 tos and placing it between two pairs of 

 porcelain cleats. A hole is then made in 

 the asbestos and a porcelain tube insert- 



C/ea/j 



This insulator for lead in wires is efficient 

 and easy to make 



ed ; the entire insulator is then nailed to 

 the roof and is ready for use. The draw- 

 ing shows the construction in detail. 



International Conference at 

 Washington 



INTERNATIONAL conferences on 

 radio telegraphy were held at Berlin 

 in 1903 and 1906 and in London in 1912. 

 The next is to be at Washington, D. C. 

 The regulations adopted have been 

 agreed to by most of the countries of 

 the world. 



Radio Has Velocity of Light 



IN THE experiments between the 

 pow^erful Navy station at Arling- 

 ton and that of the French government 

 at the Eiffel Tower, Paris, which were 

 carried on two years ago, it was found 

 that the velocity of electromagnetic 

 waves as used in radio was substan- 

 tially identical with the speed of light. 

 The measurements were made by tak- 

 ing carefully timed photographic rec- 

 ords of signals sent across the Atlantic. 



