Page:Popular Science Monthly Volume 88.djvu/815

 Popular Science Monthly

��787

��Audion of Increased Sensitiveness

���r

��New Audion Bulb

��"N the attempts to increase the sensitiveness of audion detectors and amplifiers, there have been devised many in- ternal arrange- ments of the three usual elements (grid, plate and fila- ment). In some, the spacing of the con- ductors is changed, and in others the forms of the electrode are radically different from those usually encountered. A type of tube shown in the diagram, was patented by A. McL, Nicolson in 1915. It is illustrated in the diagram accompanying specification No. 1,130,009. This instrument con- tains within its evacuated bulb two concentric cylindrical electrodes, of which the inner i corresponds to the grid and the outer 4, to the plate of the ordinary audion. The filament 2 is wound spirally around the inner elec- trode in a groove 3, but is, of course, insulated from the surface supporting it. The construction shown is said to show increased efficiency because the filament is placed close to the input electrode i and because this electrode is of large surface; these two conditions co-operate to set up a strong electro- static field between the filament and the grid or its equivalent, and this has been found to make for increased amplification.

Constructing a Variable Condenser

A VARIABLE condenser, cheap and easily made, requires the following materials: tinfoil, 2 ft. of }i-\n. ash or oak, and 9 plates of glass, such as old photographic negatives. The plates should be 2>% ins. by 4>i ins.

Cut 18 rectangles, 3^ ins. by 2^i ins., of tinfoil. Shellac them at the center of both sides of the glass plates. Care must be taken that the foil forms an even coating on the glass. Next, con- struct a box of 5 sides, having the following dimensions:

Top 4^8 ins. by 2}i ins.

Bottom Syi ins. by 2}< ins.

��Sides ^yi ins. by 3 ins.

Back 2% ins. by 3 ins.

Before assembling the box, make 9 cuts lengthwise across the top and bottom, 3/16 in. apart and H in. deep. The first groove on the top and also on the bottom should be made }4 in. from the edge. These grooves will be the correct width to hold the plates firmly in place, if made with a rip-saw. The 2nd, 4th, 6th and 8th grooves from either side must be somewhat wider than the others so as to allow the plates to slide along easily. The top and bot- tom of the box are glued and nailed to the sides, as shown in Fig. i.

The back is put on after clips have been fastened to the ist, 3rd, 5th, 7th and 9th plates and a wire soldered across them, as shown in Fig. 2. The

G/ass r/nibif

fii/dder /}(7/x/^ 'cross p/ece

���F/g.l

��Stop d/ocA

��F7h~

��^

��6/aJJ p/afes

����f/g.2

A variable condenser can be made with old

photographic plates and tinfoil. These

diagrams show construction details and

the necessary connections

second, 4th, 6th and 8th plates are also to be connected together in thd same way.

A wooden stop-block, measuring % in. by yi in. by 2 ins., must now be made and fastened }/s in. from the edge of the end of the bottom, as shown in Fig. I. A handle of hard rubber should be fastened to a cross-piece, measuring }4 in. by }4 in. by 2 ins., and having 4 grooves like the ones first made, but Ys in. apart. The first should be 7/16 in. from the end.

The terminals should be made of spring-brass less than 1/32 in. thick and bent as shown in Fig. 3. One touches the tinfoil of the first plate: the other touches the foil of the eight plate, as

shown. — W. E. FiNKERNAGEL.

�� �