Radio broadcast .. (1922-30)

Rheostats for the Tubes You Use How to Select the Proper Rheostat. Operating Different Kinds of Tubes From the Same Battery. Using Tubes in Series and in Parallel. By ZEH BOUCK T "AHE advent of the dry-cell tube, while making radio a more simple and economical proposition, has by chance complicated matters when enthusiasts have attempted to operate such tubes from voltages other than those for which they have been designed — a possibility that is often made desirable by the possession of a six-volt storage battery. Similar difficulties are encountered when it is endeavored to light different makes of dry-cell tubes, with their individual filament requirements, from the same battery, and haphazard experiments by many fans have resulted in blown tubes. Such disasters, however, would have been obviated by an understanding of perhaps the most fundamental of electrical axioms, Ohm's law, and its application in solving the problems of the proper battery and rheostats. Ohm's law is a statement of the three predominant characteristics of an electrical circuit, the voltage, current, and resistance, and the limitations imposed by any two of these qualities upon the remaining one, i. e., their inter-relation. In the pioneer days of electricity, helped along by the fluid theory of electric currents that then prevailed, physicists were led by observations to the correct conclusion that electrical phenomena possessed two characteristics, voltage and current, the former being the pressure that sent the current through the wire, and the current itself being the density or heaviness of the electrical stream. It was found that this last quality, current, depended on two things, first upon the voltage or pressure and secondly, upon the resistance of the conductor or wire. As the pressure increased, it was quite natural that the current would become heavier; as would a lazy stream of water in a pipe when the force applied to the reservoir end was multiplied. It also followed, that as the resistance of the conductor was made greater, for instance by using a smaller wire, the current was decreased, as would be the flow of water in the aforementioned pipe were a plumber to substitute a smaller diameter pipe or place some obstruction (resistance} within it. This law of electric currents is expressed in the equation, E I= — R I, E, and R mean, respectively, current in amperes, potential or pressure in volts, and resistance in ohms. Hence we might substitute for the abbreviations or symbols, and state the equation, potential current = resistance or, still another way, amperes = volts ohms By obvious algebraic transpositions, the two follov/ing equations are simply derived from the original statement: E R= — for finding resistance and E = I x R for finding voltage or "voltage drop." These equations are of inestimable value in all electrical work, and are particularly applicable, as will be shown, to the calculation of filament resistances and voltages. WHAT SIZE RHEOSTAT SHALL I USE? THE accompanying chart, Fig. i, shows the three laws, and indicates the various computations in which they are commonly involved. The equation most readily applicable to the operation of the dry-cell tube is the second, and which declares that the resistance is equal to the voltage divided by the current. We shall presume that the reader is interested in the construction of a portable set with detector and one step, using the UV-I99 tubes. Referring to Fig. 2 (or to the circular accompanying the UV-I99 when it is bought), it will