Questions and Answers

Radio Record, Volume II, Issue 20, 30 November 1928, Page 11

Questions and Answers

EADERS are invited to submit to "Observer"? questions that can be answered in this column. The questions should be short and concise and capable of a reasonably short answer,

. 2FB, Northland.-iIf you were to try Thos. Ballinger you would probably be able to obtain 260 feet of bell wire in one piece. The collars you could no doubt make, G. M. Tiffen (Gisborne).-We do not know the make of receiver to which you refer. If you can submit anything in the nature of a diagram we shall be only too pleased to help you if we can. Introducing a Voltmeter. ad has often been said that it is practically essential to place a voltmeter permanently in the circuit to ensure the correct working of valves, more so when dry batteries are used, but there has been an absence of information as to how and where it should go. "I would be pleased if you could supply the following information :- (1) Just where in the circuit should a meter be connected to read the voltage across all the filaments? (2) What alterations, if any, would be necessary ? (3) Would a high grade instrument such as a Weston detract from the efficiency of the set? ° "The set,in use is the BrowningDrake described in ‘Record. Valves have been a very expensive item to me, and I now wish to give them every chance of a long and useful life. Before closing, may I express my appreciation for your corner and wish it every success."--I'. LR. (Christchurch). Connect the voltmeter across the filament terminals of the valve sockets themselves. These are the two thicker valve pins. The meter can then be permanently fixed on the circuit. The current on the valves could then be read irrespective of the battery voltage. The Weston voltmeter, consuming as it does but little current, would be a safe proposition, and would not affect the set. An inferior one would be decidedly unsafe, Length of Aerial. "SN7HAT is the maximum length of aerial and lead-in on which 2YA ean be tuned? How can this be calculated?" Wach aerial has a certain natural frequency or wave-length. A station operating on this wave-length could be received without tuning devices, that is

without a coil and condenser. The object of these latter is to tune the aerial or the set to the frequency of the signals desired. A station cannot be tuned in if its wave-length is shorter than that of the receiving aerial, without special’ devices. The exact naturai frequency of an aewial is not easy to caleulate without mathematical experience, but the following is a general method suggested by "Radio News." Add together the length in feet of the aerial, the lead-in and the ground wire, then multiply the total length by 1.5. For example, if the aerial is 100 feet in length, the lead-in 40 feet and the ground wire 20 feet, the total length is 150 feet. Multiplying this by 1.5, the result is 225, the natural period or wave-length of the aerial. Applying this, it will be found that the length of aerial above which 2YA. cannot be received is 280 feet. In practice, except in the case of crystal sets, an aerial this length would be far too long; it would pick up rather much atmospheric noise, although the signal strength would be much greater than, say, with 100 feet. Mushiness From Speaker "[ HAVE a erystal set using a permanent detector; this is resistance: coupled to a one-stage amplifier with a valve requiring 90 volts on the plate (B). I am using a small speaker of a popular make. Speech and high musie come through O.K. but as soon as anything heavy or mixed, such as full band or orchestral music or choral items comes on, the speaker blurs as if the magnets were too close to the diaphragm (speaker rattle). The same effect is noticeable when the speaker is connected directly to the erystal while the earphones give perfect reproduction." This latter clause seems, as suggested by the writer, to point to the’ speaker and to rule out the amplifier. As this outfit is in use near to 2¥A (Pipitea Street), it would seem that the fault is with the speaker, which is not capable of handling the volume, and the lower notes, as it takes a better speaker to handle these than the higher registers. Most speakers ‘will handle the high notes fairly satisfactorily, but it takes a good speaker and amplifier to handle satisfactorily volume and low notes. A good cone is usually required.