The Birmingham age-herald., 1922-10-08
RHEOSTAT MOUNT
 IS PORTANT IN THE RADIO RECEIVER
Standard Makes Usually De- signed to Carry Current for One Tube-0verheat
 When More Used
By JACK BINNs

The filament rheostat Is frequently a source of trouble in vacuum tube circuits, and too often the article Is condemned because of breakdown under conditions that might easily have been avoided filament srheo- stat which functions properly must maintain a constant resistance for any position to which the contact arm is turned, must be capable of carrying it's rated current without overheating, and finally must be so constructed mechanically that it will not break down under the tempera- ture conditions to which 1t Is liable to be exposed.
 To a certain degree the manufac- turer is responsible for the produc- tion of a device that will fulfill all three of these requirements, but. on the other hand, the man who buys the rheostat for installation In his set must use discretion in arranging it in the circuit and mounting it.
 Heat Increases
 In the first place it- must be rec ognized that the ordinary filament rbeostat is designed to accommodate only the current for one tube. If, for example, in an amplifying series one rheostat is used to carry the current for two or more tubes the temperature rise in the resistance coils is liable to be excessive. The heat dissipated in a resistance unit varies as the square of the current. so that if twice the current Is passed through the same rheostat the heat produced tand therefore, tempera- ture rise) will be four times greater than 1t was when used with only one tube.
 As long as current Is passing through the rheostat heat will &C., cumulate in the resistance wires and mountings until these parts reach a temperature sufficiently high to give the heat off to the surrounding air just as fast as 1t is being produced. Air circulation therefore plays an important part in relieving such a situation, and consequently the rheo- stat should be mounted in such way as to take advantage of this action. Since heat rises the rheostat should not be mounted inside the top of a shallow cabinet, nor should the coils be enclosed in a small space for the sake of appearance.
 Careful Mounting Needed
 The best mounting is on a vertical panel sufficiently clear of other ele- ments composing the circuit. This produces a continual free circulation of air upward past the coils, thereby decreasing the probability of over heating. An improperly mounted rheostat may give no trouble until some time when accidentally or oth- erwise it's resistance is turned all the way in for half an hour or so.
 One of the most frequent sources of trouble with the filament rheostat is however, due to improper assem- bling. Firm contact of the spring arm bearing against the coils is ab- solutely essential for satisfactory op- eration of the set and long life of the rheostat. Disagreeable noises are frequently traceable to slight va riations in the filament current when vibration of the set changes the pres sure of a loose contact arm. A very small change in the pressure between two metallic surfaces when the nor nal pressure is small will produce marked changes in current flow be- tween them.
 Not only is such laxity in assem- bling the rheostat liable to reduce the efficiency of the set, but 1t will materially shorten the life of the re- sistance coil. Loose contacts cause excessive overheating at the point where the current passes from the contact arm to the coil and the final stages are accompanied by arcing, fhe effect of which is accumulative eA hastening the unfortunate end.
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