Radio Broadcast (May-Oct 1922)

358 Radio Broadcast of energy is accomplished with an efficiency of not more than yio, owing to hysteresis and eddy currents. (These scientific terms apply to magnetic losses which occur in all electromagnetic devices). Telephone receivers used for radio purposes give an audible signal with a minimum current of from i to lo thousandths of a micro-ampere at a frequency of i ,000 cycles per second. The relative sensitiveness of different types of telephone receivers is characterized practically by their ohmic resistance, in practice, however, it is not always advantageous to employ receivers of high resistance, according to this authority, since extreme sensitiveness is a defect if selectivity with signals of varying intensity and notes is desired. Efficiency in reception depends in part on the employment of telephones of resistance appropriate to the detector used. For the same amount of received energy, a crystal de tector (average resistance 8,000 ohms) produces in the receiver a current of low intensity at a high voltage, while a magnetic detector (resistance 140 ohms) gives, on the contrary, a current of low voltage and of high intensity. In the case of high-resistance receivers, the best results will be obtained when working with crystal detectors or vacuum tubes, and with low-resistance receivers, the best results will be obtained with magnetic detectors and other low-resistance detectors. (The magnetic detector, which makes use of a soft iron core wire band, passing continuously through the. magnetic field of a pair of permanent magnets and a winding connected with the telephone receivers, was employed several years ago in radio communication, prior to the introduction of crystal detectors and vacuum tubes which are simpler, yet more sensitive to signals.) 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 Names, however, will not be published. Variable Condensers / have a 4^-plaie variable condenser and a receiver with two variometers. May the condenser he employed to advantage with this receiver? — M.M.. Detroit, Mich. A 43-PLATE condenser of the commercial type orZ-\ dinarily approaches .001 mfds capacity. A ca' »■ pacity as high as this is rather difficult to employ in short wave work, although it is very satisfactory for use in conjunction with long wave reception. However, where a large antenna is employed for short wave reception, a condenser of this character connected between the antenna and tuner or the tuner and the ground reduces the natural period of this circuit and permits the use of more turns in the antenna circuit. The increased number of turns in this circuit is frequently accompanied by a greater transfer of energy to the secondary tuning circuit resulting in louder signals. A 43-plate condenser causes a material change in the wave lengths for a very slight change in the position of its plates and is, therefore, rather critical. As a general rule any well-made variable condenser in the antenna circuit permits more selective tuning and better all-around results, though it is sometimes accompanied by a slight reduction in signal strength where a small antennna is used. Is it possible to secure better results by using a variable grid condenser and what si^e should such a condenser be? Should a grid leak he employed with it? If so, what resistance should it he? — D.B., Trenton, N. J. AS A general rule variable units in all radio circuits aid ZA in obtaining better results because they afford a ready I V means for bringing the circuits into the most suitable balance; that is, for a given frequency or wave length best results may be obtained by employing a certain amount of inductance and capacity. Changing the inductance or the capacity may result in bringing the circuit in tune with a given wave length but its power of selection as well as its energy-absorbing values are found to exist in the greatest degree when a suitable balance of inductance and capacity is found. A variable grid condenser helps to make this balance possible in the grid circuit and it offers a convenient method for making up the differences found to exist in