Radio broadcast .. (1922-30)

The Grid QUESTIONS AND ANSWERS The Grid is a Question and Answer Department maintained especially for the radio amateurs. Full answers will be given wherever possible. In answering questions, those of a like nature will be grouped together and answered by one article. Every effort will be made to keep the answers simple and direct, yet fully self-explanatory. Questions should be addressed to Editor, "The Grid," Radio Broadcast, Garden City, N. Y. The letter containing the questions should have the full name and address of the writer and also his station call letter, if he has one. Names, however, will not be published. Technical Terms Used in This Month's Grid Accumulator: A storage battery. Zero-center meter: A direct-current meter having the %ero at the middle of the dial, and reading to both right and left. In the case of a battery ammeter, one side will indicate the charging amperage and the other the rate of discharge. Gassing: The "boiling" or excessive bubbling of a battery during the last period of charge. Open circuit: Tested when the charging current is off, and no current other than that actuating the meter is being drawn from the battery. Hydrometer: An instrument for measuring specific gravity. Flux (in electricity}: An energetic field of magnetic lines of force such as move the armature on a door-bell or the diaphragm of a telephone receiver. Inductance: The property of a circuit which determines its ability to build up a flux. A coil of wire is also frequently referred to as an "inductance." STORAGE BATTERY CHARGING / have a 1 2O-ampere-hour "A" battery, no volts D. C. is available. How can I charge the battery? L. A. H., LAINSBURG, MICHIGAN. STORAGE batteries are very readily charged from 1 10 volts, direct current, by placing a resistance in series with the battery and line. This resistance should be of such a value that it will reduce the current flowing through the circuit to the amperage designated as the charging rate, which is generally specified on the nameplate of the battery by the manufacturers. The charging rate varies with the capacity of the cells, from three or four amperes for the 4o-ampere-hour accumulator, to ten or fifteen amperes and higher with the larger batteries. For a i2O-ampere-hour battery, an 8-ampere charging rate should be maintained from 15 to 20 hours, depending on the original degree of discharge. The most convenient form of resistance is probably the electric light bulb, and jo-watt Mazdas, or i6-candle power carbon lamps should be used, except in the case of large batteries where doubling the power of the lamps will halve the number. One 5O-watt bulb should be used for each half ampere — or a loo-watt lamp for each ampere. Thus, charging a i2o-ampere-hour battery, at an 8ampere rate, will require 16 5O-watt bulbs. The lamp sockets, and at least two switches, should be mounted on a hardwood panel, which is most conveniently installed close to the battery. The necessary switches are the line switch with fuses, and the battery, double pole double throw, charge-discharge switch. Two additional switches which add to the utility of the panel and facilitate operation are indicated in the diagram, Figure i. An ammeter of the automobile zero-centre type, reading up to fifteen amperes in both directions is also desirable. Switch B makes it possible to halve the charging rate, as should be done during the last two hours of charge, without unscrewing eight uncomfortably hot lamps. With switch A open, the filament battery is placed on "trickle charge," to which the battery should be occasionally subjected, and which will compensate for surface leakage during comparatively long periods of disuse. It must also be born in mind that, while it is theoretically possible to dissipate all of the energy put into the battery, some of the energy is not available for useful purposes, a fact that necessitates a slight overcharge which is most safely effected by frequently placing the battery on an overnight trickle charge. Also, with the charge-discharge switch open, or in the discharge position, the " B" battery, up to an 8o-volt potential, may be charged from the binding posts X and Y at the half-ampere rate. Charging of the "A" and "B" batteries must be accomplished separately, but either may be charged while the set is in operation, if the receiver is of the inductively coupled type, i. e., with no wired connection between the antenna-ground circuit and the audion. In all other hookups, the batteries must be disconnected from the set during charge, a precaution most conveniently accomplished by charge-discharge switches. In charging, the positive of the line must be connected to the positive pole of the battery, so it will be probably necessary to test for polarity. Four methods are in general use, and are indicated in order of their simplicity: 1. By feeling for shock. 2. By an electro-chemical polarity indicator. 3. By a direct current, magnetic type voltmeter 4. By the decomposition of salted water. In almost every case the negative of the no-feedis grounded. Therefore if the fingers of one hand are placed on the positive wire, and those of the other hand grounded on the steam or water pipe, a gentle shock will be felt. Contrary to general supposition, a i lo-volt shock through dry hands is very mild, and a pleasant sensation rather than otherwise.