Radio Broadcast (Nov 1926-Apr 1927)

504 RADIO BROADCAST ADVERTISER as * usuai Whenever a new set or circuit is good you are practically certain to find Centralab Products specified as part of it. Used in the DAVID GRIMES CIRCUIT M CENTRALAB n ODULATOK for volume control — takes the "rough spots" out of volume — smooths out powerful "locals" as well as "DX" — and prr> vides noiseless control of tone volume without in any way affecting the tuning of the set. Has a maximum re sistance of 500,000 ohms, specially tapered to give smooth, even control from a whisper to full volume — or vice versa — without de-tuning. $2.00 at your dealer's or mailed C. 0. D. Centralab Radiohm A two terminal variable high resistance that is standard on many leading sets. Gives noiseless control of battery voltages or of regeneration. Furnished in a range of resistances from 2,000 to 500,000 ohms, all variable to sero, and in standard or heavy duty types. CENTRAL RADIO LABORATORIES 22 Keefe Ave., Milwaukee, Wis. Canadian Representative — Irving W. Levine, Montreal Australian Representative United Distributors, Ltd., Sydney Great Britain Representative R. A. Rothermel, Ltd., London No. 78 Radio Broadcast Laboratory Information Sheet March, 1927 The Volt, Ampere, and Ohm DEFINITIONS ^JS7E ARE giving below an explanation and ^ » meaning of the common terms, the volt, the ampere, and the ohm. Hydraulic analogies will be used in explaining the first two of these terms. Ampere : A current of water in a pipe is measured by the amount of water that flows through the pipe in a second, such as 1 gallon per second, or 10 gallons per second, etc. Electricity is measured by the amount of current that flows along a wire in one second. This quantity is known as the coulomb, and if this term is used we would express the current as 1 coulomb per econd or 10 coulombs per second, etc. In electricity, however, we have a special name for the rate of flow of 1 coulomb per second which we call 1 ampere. Thus, 8 amperes is the same as 8 coulombs per second. Ampere, then, is a term defining the quantity of current that is flowing per unit of time. Volt: The number of gallons per second of water flowing in a pipe, or the number of amperes flowing in a wire, depends upon the pressure under which it flows. The electrical unit of pressure is the volt. A volt means the same thing in speaking of a current of electricity that a pound pressure means in speaking of a current of water. It follows then that the greater the pressure (voltage) at the supply, the greater will be the flow of current. Ohm: There is no hydraulic unit which corresponds to the ohm, which is a measure of the resistance of a wire to the flow of current. A wire is said to have 1 ohm of resistance when a pressure of 1 volt will cause a current of 1 ampere to flow through it. If the resistance were doubled, the current would be halved, etc. According to the definitions given on this Sheet, then, we see that amperes represent the amount of current, volts the pressure causing this current to flow, and ohms the resistance impeding the flow of current. These three units bear a definite relation to each other. This relationship, named after the scientist who discovered it, is known as Ohm's Law, which states that the number of amperes flowing in a circuit is equal to the voltage of the circuit divided by its resistance. An explanation of Ohm's Law is given on Laboratory Sheet No. 81. Radio Broadcast Laboratory Information Sheet March, 1927 Regulating Voltage on B Power-Supply Device USE OF RESISTOR "jV/rANY commercial a. c. operated power-supply A" devices are equipped with taps for supplying different voltages suitable for use in conjunction with the detector, amplifier, etc. The voltage obtained from any tap on such a device is not constant but varies with the amount of current that is drawn from it. If an unusually heavy load is drawn from any one of the taps, it will generally be found that the voltage is somewhat less than the specified amount. In such a case, it is possible to increase the voltage on the particular tap which is low by connecting an external resistance between the tap whose voltage is low and the maximum voltage tap on the device. The proper connections for this resistance are indicated in the diagram, and, by the proper variation of this resistance unit, it wiU be found possible to obtain any value of voltage that might be required. This method of increasing the voltage on any tap is very simple, since it does not require that the power-supply device be opened and the internal resistances varied. It should be noted that the resistance does not connect between the two adjacent taps but that it is connected between the 45-volt tap and 135-volt tap which, in this particular case, is supposed to be the maximum voltage tap on the device. This method of increasing the voltage on any tap was suggested Power-Supply Device To Set in the December issue of the General Radio Experimenter. The resistance should be variable between 5000 and 50,000 ohms, and must be of a type satisfactory for use in power-supply devices. Radio Broadcast Laboratory Information Sheet Characteristics of Tubes March, 1927 MEASURING THE AMPLIFICATION CONSTANT ¥ ABORATORY Sheet No. 68 (February, 1927) ^ gave some characteristic curves of the 171 type tube, and Sheet No. 67 explained how the plate impedance of the tube might be calculated using these curves. The present Sheet will explain how to calculate the amplification constant. The amplification constant is the measure of the effect of the grid voltage on the plate voltage. Stated as a formula, the amplification constant equals: CHANGE IN PLATE VOLTAGE CORRESPONDING CHANGE IN GRID VOLTAGE We are giving two examples below which will make simple the calculation of the amplification constant of any tube provided its characteristic curves are available. Example 1 : Calculate the amplification constant of a 171 using the curves given on Laboratory Sheet No. 68. In this example we shall use curves Nos. 2 and 3. Locate some point on curve No. 2; in this example we are taking the point corresponding to 100 volts, although any point might be taken provided we stay on the straight portion of the curve. We find that at this point, corresponding to 100 volts, the plate current is 12.5 milliamperes. Following across the horizontal line corresponding to this plate current until we come to curve No. 3, we find that the corresponding plate voltage on this curve is 128. We now have two voltages, 100 and 128, corresponding to two different grid biases, 16.5 and 27. Both of these values are for the same value of plate current. These values can be substituted in the above formula as follows: 128 — 100 28 10.5 27 ■ 16.5 Solving this formula, we get a value of 2.67, which is the amplification constant of this particular 171 type. Example 2: Find some point on curve No. 4, taking that point corresponding to 160 volts as an example. In this case a plate current of 20.3 milliamperes is obtained. Following across to the corresponding plate current on curve No. 5 we find that the plate voltage is 179. The difference in plate voltage between these two points is 19 and the difference in grid bias is 40.5 minus 33, or 7.5. Dividing these two, we obtain a value of 2.54, the amplification constant of the 171. It should be noted that this latter result is somewhat different from that given in the preceding example due to the fact that the tube was considered to be operating under different voltages. The amplification constant varies slightly for different plate voltages, but the variation over the operating range of plate and grid voltages is not usually more than 10 per cent. J