AERIAL DANGERS

Sun (Auckland), Volume III, Issue 640, 17 April 1929, Page 15

AERIAL DANGERS

i VALUE OF EARTH SWITCH When first they install their receivers, most listeners ■watch each thunderstorm which occurs in their district with a good deal of apprehension, fearing that a lightning discharge may strike their aerial, destroying their receiver and possibjy setting fire to the house. Experience has shown that the lightning hazard is in no way increased by the presence of a wireless aerial, and that the danger from direct stroke is certainly not greater than in oases in which a telephone or elec-tric-light service is installed. The number of cases in which direct strokes on to the aerial has occurred is very small. As a result of this many listeners grow inclined to disregard a very real danger to the receiver which does exist in stormy and thundery weather, and invite damage to their equipment by failure to leave the aerial effectively connected to earth. The danger to a receiver from an unprotected aerial is by no means confined to the actual period of a thunder-storm. It exists to a greater or less extent throughout all sultry and dry weather. The reason for this is that even when direct lightning discharges are not visible the atmosphere can attain a highly electrified condition, and if it be well insulated, and not effectively earthed, substantial charges at high pressure can collect on a broadcast receiving aerial. If the tuner used consists of a primary coil to which

aerial and earth wires are connected, the danger from these accumulated aerial charges is greatly reduced, as they are not always of an oscillatory character, and can then escape slowly to earth through the tuning coil. If. however, a series tuning condenser in the aerial lead is used, no direct path to earth is provided. The aerial charge will then grow until the potential reaches a value great enough to discharge to some earthed part of the set by means of a direct spark. This charge can be very destructive, because it is oscillatory in character, and will not normally pass through tuning coils. It will escape by means of a .series of sparks which may do serious damage to insulation, and by destroying the insulation in a vital part result in the short circuiting of the high-tension battery. A case came under notice a few days ago in which such a discharge apparently occurred in a receiver. The set was a short-wave receiver employing a screened grid valve as a radio-fre-

quency amplifier. The intensity of the final discharge when the “flashover" in the coupling condenser occurred was so great that the glass of the radio-frequency amplifying valve was fractured.

Most broadcasting receivers are, or should be, fitted with a lightning arrester permanently connected in place. It is a good plan, however, to provide, in addition to this arrester, a switch by means of which the aerial can be permanently “earthed" when not in use. The lightning arrester consists of a small spark gap, s.cross which a discharge can spark without passing directly through the receiver. It is possible, however, in the final stage of the discharge, which is oscillatory, for considerable potentials to be set up across the primary tuner in the receiver. These potentials may induce still higher ones in the other tuning coils, breaking down blocking condensers, and damaging leaks and high resistances. An earthing switch will prevent a spark discharge in the arrester, and eliminate all possibility of damage.

EMERY PAPER

Emery paper is so useful an article to the radio amateur and constructor that it is a matter for surprise that very few' workers ever attempt to make their own supplies of this commodity. The process of its manufacture is a simple one, however. Take a small quantity of good glue, and prepare it in the usual manner. While the glue solution is still hot, add to it a quantity of fine emery-powder (or powdered glass), so that, on stirring, a grey or black liquid is obtained.

Spread this prepared glue lightly and rapidly over the surface of thick brown paper, and then set aside the sheets of paper to dry. The result w r ill be an abrasive paper of good quality. Of course, any of the glue preparation which has been unused may be set aside and employed for the preparation of further supplies of the paper on future occasions. The preparation will keep indefinitely.

NEW POWER VALVE

A new power amplifier valve for supplying large undistorted output to the loud-speaker, was announced recently by the Radio Corporation of America, and E. T. Cunningham, Inc. The 245, as the valve is designated, is said to be capable of delivering a power output equal to that of the UX-210, but a plate voltage not exceeding 250 volts. It is not interchangeable with the 171-A or any other power amplifier. The valve can be used only with apparatus especially designed for it, and is intended for use in the last audio amplifying stage of power line operated sets which supply not more than 2.5 volts to the last audio socket, as •well as proper grid and plate voltages. The filament is of the coated ribbon type, which assures strength and high emission. To keep the exceedingly high plate current from the loud-speaker windings, it is essential to use some form of loud-speaker coupling, such as an output transformer or a choke coil and condenser.