568 
cations of these, but we may mention that we notice 
among the designs some convenient ‘‘twin'’’ and 
“universal” patterns which enable a large range of 
resistance and accurate regulation to be obtained with 
apparatus of smaller dimensions than a simple rheo- 
stat would have for the same requirements. The 
catalogue contains a diagram of the “ Ruhstrat” 
winding, designed to eliminate, so far as possible, 
Fic, 1 —Ruhstrat 
Fic. 2.—Ordinary 
winding. 
Fic. 3-—Chaperon 
double winding. i 
winding. 
capacity and safe induction. Two wires are wound 
in parallel in opposite directions and symmetrically, 
as shown in Fig. 1, which may be compared with the 
ordinary double winding shown in Fig. 2, and the 
Chaperon winding in Fig. 3. 
The idea used to prevail that practically anything 
would ‘‘do”’ to insert as a resistance for reducing 
the voltage of the supply mains for experimental 
work or various miscellaneous purposes. This has 
resulted in trouble from time to time, and more care 
is now exercised in the selection of suitable pieces of 
apparatus. One form of rheostat, for instance, which 
was at one time extensively used in connection with 
medical applications of electricity, has a particular 
element of danger. It was customary to use a coil of 
very fine wire wound on slate connected as a potentio- 
meter between two supply mains, as shown in Fig. 4; 
an apparently small and easily adjustable difference of 
potential is thus obtained between the points A and 
B, but the danger is 
that if the part of 
+ _ = * 
the rheostat wire be- 
pee EE OVS ae tween these two 
points should burn 
out, the pressure be- 
tween A and B will 
suddenly rise to the 
full 220 volts. 
Another method of 
applying such a 
rheostat is shown in 
Fig. 5 in connection 
with an electric bath. 
The bath, of course, 
is porcelain. The 
main resistance in 
the circuit in this 
case is the lamp B, 
so that the current 
passing through the 
bath can be adjusted 
with considerable accuracy by moving the slider along 
the rheostat, and in ordinary circumstances the poten- 
tial difference between the two electrodes in the bath 
is not high. It must be remembered, however, that 
if the two points A and D are connected across the 
supplv mains, one of these will be at earth potential. 
If this is the point A it is evident that the point D, 
NO. 2307, VOL. 92] 
Fheostat 
WDOOONDOOOODONDD DONO 
To Electrodes or Bath 
Fic. 4. 
NATURE 
. 170X20=3400 watts, or about 43 h.p. 
[JANUARY 15, 1914 
which is directly connected to one of the electrodes, is 
actually at 220 volts potential, and a patient sitting 
in the bath and touching a Water tap would imme- 
diately receive a 220-volt shock. If the connections 
between the rheostat board and the electric lighting 
mains are made through an ordinary two-pin plug, as 
shown in the diagram, it is an even chance whether 
this wrong connection is obtained or not. A similar 
shock could be obtajned, no matter what was the 
polarity of the connections if the lamp were to become 
short circuited. 
For heavy currents proper precautions must also 
be taken in the construction of rheostats. If, say, a 
20-ampere arc lamp for a high-power lantern projector 
is used from a 220-volt main, it necessitates the use 
of a resistance which shall cause a drop of about 
170 volts, i.e. the consumption of power of 
If wire of 
sufficient thickness is not used, this will rise to a 
very high temperature, and, no matter what the 
gauge of the wire, it is clear that if the lamp is used 
for a long period at a time a very large amount of 
heat has to be dissipated. The wiring rules of the 
---+------~---------~-4-220/--------------- 
Rheostat 
Fic. 5. 
Institution of Electrical Engineers, which unfor- 
tunately are not always observed in such cases, limit 
the maximum temperature of the case containing the 
resistance to 176° F.*(80° C.). The rules also specify 
that the resistances must be enclosed in cases of in- 
cembustible material, or protected by wire gauze or 
perforated sheet metal, and fixed so that no unpro- 
| tected combustible material is within 24 in. vertically 
above the case, or within 6 in. in other directions. 
Largely owing to the development of apparatus for 
electric cooking, considerable progress has also been 
made in recent years in the composition of the wire 
itself. For cases in which large currents are em- 
| ployed, wires made of special alloys have been pro- 
| duced which will stand a very high temperature with- 
| out oxidising or becoming brittle. It is true that when 
a constant resistance, practically independent of tem- 
perature, is required, such alloys may not always be 
suitable, but for heavy current work, such as indi- 
cated above, they can naturally be used with consider- 
able advantage. In the mechanical construction of 
resistances, great progress has also been made. 
