Page:Popular Science Monthly Volume 88.djvu/333

 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.

�� �