Radio Broadcast (May-Oct 1922)

360 Radio Broadcast vacuum tubes. In the same circuit one vacuum tube may require a very small grid capacity for its best operation while another tube may require comparatively more capacity. With a variable grid condenser the most suitable capacity may be had instantly. The same thing applies to a given circuit and a given tube receiving from several stations. A variable grid condenser aids materially in building up desired signals and eliminating undesired signals. A variable grid condenser should be of comparatively low capacity; that is, it should have a maximum of approximately .0006 mfds. It is rather difficult to determine without actual experiment whether or not a grid leak is required in a given circuit or with a given tube. Vacuum tubes vary greatly and the function of the grid leak is to keep a constant potential on the grid of the vacuum tube in order that the electronic flow may be thoroughly controlled. Some vacuum tubes operate most satisfactorily without a grid leak. It is also significant that a tube of this character employed in one circuit would give results without the grid leak, while in another circuit, the grid leak would have to be used in order to obtain the best results. The resistance value of a grid leak is also a matter of experiment. As a general rule a grid leak resistance of 2 megohms will suffice. It is generally a safe practice to employ a grid condenser and grid leak unit of the character now on the market having a capacity of .0005 mfds and a resistance of 2 megohms. LiGHiNiNG Protection Do I need to install a ground switch for lightning protection? I am using a crystal receiving set. — N.M., Topeka, Kansas. What si^e wire must I use for grounding my receiver? Does the installation oj the receiving set cut of the insurance on my home? —K.P., New York, City. My receiving set is on the tenth floor of a city apartment. Is a necessary for me to run a heavy copper wire to an outside ground connection keeping the wire seven inches from the building wall? —j.D., Brooklyn, N. Y. UNTIL a very short time ago, the National Board of Fire Underwriters required all radio stations to be fitted with a heavy current carrying ground switch. This rule has been materially changed in the case of receiving apparatus and it is now only necessary for a receiving station to employ a suitable lightning protective device which has been passed by the Board of Fire Underwriters. According to the new Fire Underwriters' regulations recently proposed receiving sets may be grounded by any copper wire not smaller than No. 14 B & S gauge. The installation of a radio receiving set, when the provisions of the National Board of Fire Underwriters have been lived up to, should not interfere in any way with the insurance on a house. The regulations are quite easily lived up to and if you entertain any doubt as to the wiring of your receiver, it might be well to have the installation passed on by some representative of the Board. It is possible, however, that such a representative might not be informed of the recent alterations in the code of regulations and it may be necessary for you to communicate with the headquarters of the underwriters' organization. The new regulations make it unnecessary for any one installing a receiving set to employ 9 heavy lightning switch and heavy wire running to the outside ground connection separated from the building by insulators several inches long. It is now merely necessary for you to employ an approved lightning protective device connected to some permanently grounded metal system in the house or apartment, by a No. 14, or larger, B & S gauge copper wire. By a permanently grounded system is meant the steel frame of the building; the pipes of the water supply system pipes of a steam or hot heating system, but not the gas pipes. Where a connection is made to the grounded metal system, the copper wire should be attached by soldering or by an approved ground clamp after the building frame or piping has been thoroughly scraped with a coarse file or sandpaper. Lamp-Socket Antennas There are several devices on the market for use in a lamp socket to replace the outside antenna. IVill they really work or are they fakes? — F.S., Long Branch, N. J. THE devices you refer to have been rather thoroughly tried out and in most instances have proven almost as satisfactory as the average outdoor antenna. These devices are designed to cut out the noise in the telephone receivers which would be caused by low-frequency electric charges such as exist in electric light wires, while permitting the high-frequency radio charges to pass through; they are actually filters. Where the electric light wires are run under ground the results obtained are not as good as where these wires are carried on poles such as is the case in the country. Devices of this character made by reputable manufacturers should give satisfaction, but it is quite possible that the demand for them will be so great that many irresponsible makers will exploit these machines shortly. This is likely to be disastrous to many receiving sets. The device includes a condenser made in a similar manner to most other condensers where metallic plates are separated by some insulating substance such as mica, but of different capacity. Where these units are employed there is no ready means of determining which side of the lamp socket in the house is connected to the line side and which is connected to the ground side of the electric light circuit. Where a connection is made to the ground side of the lighting system, no danger exists. However, if the device is connected to the line side of the electric light circuit and for any reason the plates of the opposite sides of the condenser should come in contact with each other or the insulation between them should break down, it would be possible for a short circuit to take place, because the line wire would then be connected through the device and your receiving set to the ground. In all likelihood this would blow out the fuse in the lighting circuit but, before the fuse was blown, it is almost certain that some of the wire on your receiving set would be pretty badly burned up. For this reason care should be used in determining upon the purchase of this device, and manufacturers should subject ihese units to a severe test before offering them for sale, in fact, they should be passed upon by the National Board of Fire Underwriters. Radio and Audio Amplification IVhat is the difference between radio and audio Ireqiu-ncy amplification'? Can the former be used with a lomi speaker?