New-York tribune., 1922-10-08
Rheostat Mount
Important in
The Receiver
Standard Makes Are Tysua,l,,
Designed to Garry en-.
rent for One Tube: Owe.
heat When More Are rIsed
Need Air Circulation
Must BE Mounted Careful,,
and Contact Arm Mu.,
Be Securely Fastened
By Ralph K. Potte,
Tribune Institute Enni..4.-


The flament rheostat is f;sau.-a
source Of trouble IN .eusn ..,V -.
circuits, and too often the arss.
condemned because Of bresgS.
under conditions that might easiIyE,.
been avoided, A Alament rb.a,,,,
which functions properly must rs'.
tain constant resistance for anv,1.,
ton to which the contact arm, I,
turned, must be capable Of carrvf,,
its rated current without overbeatiro
and finally must be SO Constructed ma,
chanicslly that it will not break dor,,
under the temperature conditions n
which it is liable to be exposed
10 certain degree the manufactures,
is responsible for the Production ~
device that will fulfill all three Of tbasa
requirements, but, on the other bsnd
the man who buys the rheostat for ~,
stailatlon in MS set must use discrs.
tlon IN arranging it in the circuit and
mounting it. 'N
In the first place It must be recoc.
nized that the ordinary filament rsso.
stat is designed to accommodate oni,
the current for one tube. II. for er.
ample, in an amplfying series on,
rheostat Is used TO carry the curren.
for two or more tubes the temnara.
ture rise in the resistance coils is iiabIa
to be excessive. The heat dissinateJ
in resistance unit varies as the souara
Of the current, so that if twice the sur.
rent is passed through the same r1eo.
stat the heat produced ard. therefore
temperature rise) win be four tims,
greater than It was when used with
only one tube.
AS long as current Is pass;n,
through the rheostat heat will az,
cumulate in the resistance wires ami
mountings until these parts reach a
temperature sufficiently high to give
the heat off to the surrounding air
just as fast as it is being produced
Air circulation therefore plays an In.
portant part in relieving such aitus.
tioH, and consequently the rheosts.
should be mounted in such a way az 10
take advantage of this action. Since
heat rises the rheostat should no. Be
mounted inside the top of a shallov,
cabinet, nor should the coils be En-
closed in a small space for the sake sf
appearance.
The best mounting Is on vertizl
panel sufficiently clear of other els.
ments composing the circuit. This
produces q. continual free circulatial.
of AIR upward past the coils, thereby
decreasing the probability Of overhead
ing. An improperly mounted rheosts.
may give no trouble until some time
when accidentally or otherwise its re-
slstance Is turned all the way in isr
half an hour or so.
One Of the most frequent sources Of
trouble with the filament rheostat Is,
however, due to improper assembling
Firm contact Of the spring arm bearing
against the coils is absolutely essen
tlal for satisfactory operation OF tbs
set and long life OF the rheostat. Dis
agreeable noises are frequently trace
able to slight variations in the filamens
current when vibration of the set
changes the pressure OF loose cor
tact arm. A very small change in the
pressure between two metallic saE.
faces when the normal pressure is
small will produce marked changes IN
current flow between them.
Not only is such laxity in assembling
cheney of the set, but it will materially
shorten the life of the resistance coli.
Loose contacts cause excessive over
heating at the point where the current
passes from the contact arm TO the
coil and the final stages are accom-
panied by arcing, the effect OF which
is accumulative in hastening The IN
ortunate end.