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Page:Popular Science Monthly Volume 90.djvu/336

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320

��Popular Science Monthly

��Polarized Relay as Interrupter

��H. B. R., Monticello, Ga., inquires:

Q. 1. Please advise if an electro magnetic polarized relay will work successfully on no volt 60 cycle alternating current as a vibrator for an induc- tion coil in the following manner: I propose to use two armatures each having a contact which makes connection with a stationary contact. Do you consider this feasible?

A. I . It is believed that you will find it extremely difficult to adjust both contacts to interrupt the circuit of each coil if they are mounted on a single armature. However, by the use of separate arma- tures for each set of contacts the proposition is more feasible. Several experimenters within our knowl- edge have tried o.ut the arrangement you suggest and have reported results considerably inferior to the ordinary induction coil vibrator.

��Receiving on the Static- Coupled Tuner

J. M. S., Jr., New Castle Pa., writes:

Q. 1. Referring to the static coupled receiving set described on page 306 of the February issue of your magazine, what should be the dimensions of the coils L-i, L-2, and L-3, to make an equipment adjustable to 5,000 meters, and to be used in connec- tion with an aerial 175 feet in length, 50 feet in height, composed of a single wire.

A. 1. The fundamental wavelength of your aerial is about 350 meters and all the inductance for the antenna circuit may be included in the coil L-2. Here a coil 3^ in. in diameter, 10 in. in length, wound with No. 26 S.S.C. wire will permit the desired value of wavelength. The coil L-2 should if possible have about the same dimensions. It can of course be made smaller, and a portion of the in- ductance included at the point L-i, but we see no reason for splitting it in this manner.

Q. 2. Is a crystaloid detector as sensitive as any crystal or electrolytic detector on the market?

A. 2. Many experimenters report better results from it than with the electrolytic. For the best results the stopping condenser of the usual circuit should be one of variable capacity and possesses values up to or beyond 0.005 microfarads.

Q. 3. How shall I connect a tikker in the statically coupled receiving set?

A. 3. Connect it in the circuit at the same point as the crystal detector D. The condenser C-2 should then have rather large values of capacity, as much as .05 or o.i microfarads.

Q. 4. What wavelength does Arlington use with the undamped wave transmitter, and should I be able to hear the signals of the statically coupled receiving set and a tikker.

A. 4. By careful adjustment of the circuits to resonance with the wavelength of 7,000 meters, you should be enabled to receive the signals in your home town, but the dimensions for the coils which we have given will not permit this circuit to respond to the wavelength of 7,000 meters. Increased values of inductance must be added at the coil L-i. We would advise a loading coil of No. 24 wire at L-i

��approximately 8 in. in length by 3 in. in diameter fitted with a multipoint switch having taps about every half inch. You would obtain better results in the reception of signals from this station by using some form of the oscillating vacuum valve.

��Ground Connections

P. L. K., Somerville, Mass., inquires:

Q. 1. What effect has a long earth wire on the range of a radio set? Does it decrease the range? Does it increase the wavelength?

A. 1. The earth lead is a part of the open circuit oscillator and consequently an increase in length of it increases the effective wavelength of the entire system. It is considered preferable in all radio installations, particularly as far as the transmitting apparatus is concerned, to make the earth lead as short as possible, otherwise the range of the appar- atus will be seriously decreased.

Q. 2. I have an inverted "L" aerial 85 feet in length, 30 feet in height. The ground Avire is no feet in length. Do you consider my ground wire abnormally long in proportion to the aerial?

A. 2. For transmitting purposes your earth wire is too long, but for receiving purposes the length is not so serious.

Q. 3. What is the natural wavelength of my aerial?

A. 3. About 300 meters.

Q. 4. How may I reduce the wavelength?

A. 4. By means of a series condenser or by decrease in the length of the earth lead. It is con- sidered sufficient for receiving apparatus to connect the earth wire to water pipes inside the house.

��Sending Condenser G. D., Ligonier, Ind., writes:

Q. 1. I calculate that in order to obtain a capacity of .008 microfarads, which I am told is the maximum value that can be used at the wavelength of 200 meters, I will require 25 plates of glass 3132 in. in thickness covered on each side with tinfoil 6 in. by 8 in. Is this correct? I fear that a single bank of these plates will not be able to withstand the voltage of the transformer. Consequently would it be proper to use two banks of 25 plates each connected in series? Furthermore, would this reduce the liability of puncture?

A. 1. Each plate of your condenser will have an approximate capacity of .00066 microfarads. Twelve plates connected in parallel will give you a value of .008 microfarads. Therefore a 48-plate condenser, 24 plates in each bank and the two banks connected in series, will give you the required value of capacity. The placing of two banks in series reduces the potential on the plates to one-half the value of a parallel connection.

Q. 2. Can you suggest any method by which the plates can be coated to prevent brush discharge without immersing them in oil?

A. 2. You might cover the plates with a good grade of beeswax or paraffin wax, but the best method of course is to immerse them in oil.

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