CERTAIN FEATURES OF RHEOSTAT DESIGN 
187 
If similar computations are made for tubes of different sizes, 
loads of different resistances, and for various voltage and cur- 
rent ranges it is found that for a given voltage and size of 
tube (watt capacity) there is one and only one winding which 
will give the maximum current capacity and yet provide, in 
the case of any load, for overlapping of the series and shunt 
ranges. 
With the ordinary rheostat, rated by current capacity and 
resistance, one has no means of knowing with what voltage it 
can be used. Nor can one know whether it is capable of giving 
overlapping ranges. That these important facts have been en- 
tirely overlooked in the design of laboratory rheostats is due 
largely to the fact that the prime importance of the voltage 
as a determining characteristic has been ignored. In fact the 
laboratory rheostat has been pretty generally looked upon 
as equivalent to a resistance box of large current capacity. 
With the new design, voltage and current capacity are made 
the determining characteristics of a rheostat and the resistance 
is entirely incidental. If the windings are properly worked out 
the resulting current capacity is the greatest which can be se- 
cured with the given voltage and within these maximum limits 
all values of current and voltage down to zero may be secured, 
no matter what the load. Thus we find not only that this new 
type is easy to connect and to operate but also that it pos- 
sesses the highest possible efficiency. 
The method of mounting and the principles involved in the 
proper design of the windings may be applied to any form of 
resistance element but are particularly suitable to the sliding- 
contact tubular type. Figures 21 and 22 represent two differ- 
ent examples of the way in which manufacturers have applied 
the principles of the new design. 
Physical Laboratory, 
The State University. 
