Evening star., 1925-01-31
A Few Interesting Facts ABOUT
Resistance.

o~dinarilv resistance is looked upon
as very obnoxious element III dn
electrical circuit, because IN many
case II impairs the efficiency OF the
circuit, consumes current ana oe-
creases the strength OF received sis
nails
But lust like friction, it's counter
mart in the mechanical field, resist
ance can be a very useful element IN
the electrical field and many devices
and electrical operations would De
inpossibe IL il were not for this use
full property of many conducting ma
| terials.
Soldering irons, electric lamps, hair
curriers', toasters, electric heaters'
vacuum tube filaments, grio leah >.
potentiometers and thousands or
other electrical appliances depend on
this phenomenon as the basis or their
operation.
Different materials show marked
peculiarities in their behavior during
the passage of an electric current.
Some maintain a constant resistance
| to the flow Of current. Others have
| the property of changing their resist
ance In many cases material has
| a higher resistance when II is heated
than when it is cold, and this prop
erty of the material is taken ad-
| vantage of in the design of protective
devices to govern or control the now
! of current in a circuit.
One of the most common uses or
| this property Is in the construction
| of ballast lamps to protect vacuum
tube filaments from burning out
! through accidental cSnnection OF the
| B battery across the filament termi-
nails.
The ballast lamp is connected in the
negative lead of the 3 battery be-
tween the negative terminal of the B
| battery and the negative B battery
terminal of the set. If the B battery
is connected across the terminals OF
the tube filament the resistance of
the ballast lamp filament increases
considerably. thereby introducing a
very high resistance in the circuit
and cutting the voltage down to a
point low enough to prevent harm to
the tube filament.
A similar application Is used to
automatically regulate the filament
voltage in the filament circuit of a
tube. The amperite cartridge is an
example of this use of this phenome-
non.
The constants of the resistance ele-
ment in the cartridge are carefully de-
termined, so that with different val
ues of current the amount present in
the flament Of the tube at it's rated
capacity.
The reason why the resistance is in
creased is due to the difference in the
temperature of the fflament caused
by the greater heating effect pro-
duced by the stronger current
In this respect this type of resist
ance does not obey Ohms law, which
states that If the voltage across q
circuit is increased the current will
likewise be increased. The reason for
this, of course, is that the resistance
of the ballast lamp does not remain
constant, but increases with an in
crease of current.
Ohm's law takes into account only
such cases where the resistance is
fixed and remains constant.
When it is necessary to cut down
the number of control elements in a
receiver such lamps find very use
full place in their design.
They simplify the wiring and elimi-
nate the necessity of mounting rheo-
stats on the panel.
Resistance of the type caused by re-
sistance materials is known as ''ohmic