184 
IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
let us assume that there is available a 110-volt source and one 
wishes to pass a current of 2.5 amperes through a load of 15 
ohms resistance. A 12-ampere, 5.2-ohm rheostat of the ordi- 
nary type certainly has sufficient current capacity,- but if it 
is connected in series with the load, the smallest current which 
can he secured is 5 amperes. As it is impossible to shunt the 
rheostat across the 110-volt source without extreme, overload, 
the desired amount of current cannot be secured. 
Let us take a rheostat of the same size, wound with a wire 
of smaller gauge, so that it may be connected across the line. 
One with 18 ohms resistance, rated at 6.5 amperes, will serve. 
The lowest current which can be secured with this instrument 
in series is 3.3 amperes, which is not low enough; the highest 
current which can be secured with the shunt or potentiometer 
connection is 1.5 amperes 2 3 which is far too small. Although 
this particular instrument is able to furnish both large cur- 
rents and small currents the ranges do not come anywhere near 
overlapping and there is a large range of current which it can- 
not supply. 
Using one of still smaller current capacity, let us say a 3- 
ampere, 83-ohm rheostat, we find that it can give a current on 
series connection as low as 1.1 amperes. With shunt connection 
currents from 0 to 2.3 amperes can be obtained without over- 
loading the rheostat. With an instrument so wound the desired 
value of current can be obtained, hut the two ranges now over- 
lap by a considerable amount and it is evident that a winding 
of wire somewhat larger than that which will carry 3 amperes 
could be used. 
If we try a rheostat wound so as to have a resistance of 30 
ohms and a current capacity of 5 amperes, it is found that 
when the rheostat is in series with the load, currents from 5 
amperes down to 2.5 amperes may be obtained, and when it is 
in shunt relation currents from 0 up to 2.5 amperes can be ob- 
tained. Thus with this winding the greatest current capacity 
which is possible with overlapping ranges is secured. 
2 The rheostats which are compared are of the sliding-contact tube type. 
The tubes are all of exactly the same size and can dissipate energy at 
the same rate. 
3 When the slider is at a point distant 4.5 ohms from the full-resistance 
end of the winding the remaining 13.5 ohms is carrying its full load of 6.5 
amperes. The 4.5 ohms which is shunted with the load carries 5 amperes, 
while the load receives but 1.5 amperes. 
