in circuits of the tuned r.f. type and to 150 when used in sets of the superheterodyne type. The circuit for the grid return and necessary grid bias is shown in Fig. 1. The screen grid inclosing the plate, and connected to the prong in the base ordinarily connected to the grid in the standard tube, is led to the 45-volt tap of the same B-battery that supplies energy to the plates of the standard tubes in the set. The regular grid, which now becomes what is known as a "control" grid, is connected to a terminal at the top of the tube, and its outside connection is made as shown in Fig. 1. The plate voltage recommended is from 90 to 135 volts, preferably the latter.
Figs. 2 and 3 show cross sections of the elements in the tube. In Fig. 2, the heavy vertical line in the center represents the filament; the narrow solid spiral encircling it represents the control grid, similar in character to the grid of a standard tube. The dotted spiral that incloses both these elements represents the inner screen; the circular plate is shown broken away to expose the inner elements. The outermost dotted spiral is the outer screen. Both inner and outer screens are connected, thus shielding the plate on both sides. Fig. 3, also a cross section, shows the various elements in their relative positions. In Fig. 4 we have a suggested typical screen-grid r.f. amplifier circuit. consisting of two stages of r.f., employing the new tubes and a standard detector tube. Just what the future holds in store for this new tube remains to be seen. but experimenters are very enthusiastic about it and are looking forward to its early release. At the time this article is written no definite release date has been announced by the manufacturers, although samples have been supplied to several experimental laboratories for the purpose of circuit development.
Light-Sensitive Tube
Left, Light-Sensitive Tube: Right, Diagram for Using the Tube with One Stage of Amplification
Designed to fill the need for a rugged light-sensitive valve, or photo-electric cell, which can be used conveniently in apparatus by experimenters and manufacturers interested in television, talking motion pictures, alarm systems, color sorting, etc., this new tube replaces many of the makeshifts formerly used by experimenters. The cell is mounted in a spherical bulb, 2 in. in diameter and 5 in. in over-all length, the lead-in wires being brought out to a standard Edison screw base, as shown in the photo. The internal construction is designed to utilize all those features which are best calculated to produce a cell which is free from "spurious" or "dark" currents and from "lag" or "fatigue," in laboratory parlance. It is also said to be free from the influence of temperature and to be highly sensitive to light change over the entire spectrum. The active medium used is potassium, on the surface of which is formed a film of colloidal potassium hydride. This surface is said to have a sensitivity almost a hundred times as great as the pure potassium. Further sensitivity is obtained by the presence of neon gas. The anode is a small disk placed edgewise toward the window, so that no shadow is cast upon it. With a single step of amplification, as shown in the diagram, a deflection of 30 milliamp. has been obtained with a 100watt lamp as a light source. In this circuit a UX-112 is used as an amplifier tube with a 90-volt B-battery, 6-volt A-battery and an 8-volt C-battery. A fixed resistance of 10 meg. is used in the grid circuit. Technical information in the nature of formulæ, laws, and constants relating to specific problems will be furnished by the manufacturer.
