CERTAIN FEATURES OF RHEOSTAT DESIGN. 
ABSTRACT. 
H. L. DODGE. 
Everyone who has used rheostats of the sliding-contact type 
has experienced inconvenience in obtaining the desired values 
of current and voltage. If the rheostat is connected in series 
with the load, small currents cannot be obtained. If the load 
is shunted across a portion of the rheostat on the potentiometer 
principle, part of the winding carries a double load and con- 
sequently only a fraction of the full current capacity is avail- 
able. To secure the advantages of both methods of connec- 
tion there must be a complete rewiring of the circuit. This re- 
quires time and attention and especially in the case of students 
affords an opportunity for injury to the rheostat and other ap- 
paratus. These difficulties have been eliminated in a new de- 
sign 1 in which the line and load terminals are completely dif- 
ferentiated and properly labeled and the change from series 
to shunt connection is made by closing a simple knife switch. 
Figure 20 shows the manner in which this result is accom- 
plished. If one traces the circuits, he finds that when the 
switch is open the load is in series with that portion of the 
winding to the left of the slider. When the switch is closed 
the load is shunted across that part of the winding to the right 
of the slider and at the same time the entire winding of the 
rheostat is connected across the line. With the switch open, 
values of current up to the full capacity of the rheostat may 
be secured ; with it closed small values of current and voltage, 
down to zero, are obtainable. ' 
The importance of these features is at once apparent but 
ease of connection and manipulation mean but little if not had 
in connection with a resistance element so designed that the vol- 
tage and current ranges overlap for any load, no matter what 
its resistance. The inadequacy of the ordinary rheostat in 
this respect is a matter of common experience. For example 
1 A complete description of the new rheostat and of the various applica- 
tions of the principles involved may be found in U. S. Patent No. 1,195,- 
660, Aug. 22, 1916. 
