i879-] 
( 255 
PROCEEDINGS OF SOCIETIES. 
Royal Society, January 23. — A paper by C. William Siemens, 
D.C.L., F.R.S., “On certain means of Measuring and Regulating 
Electric Currents,” was read. It is well known that when an 
electric current passes through a conductor, heat is generated, 
which, according to Joule, is proportionate in amount to the 
resistance of the conductor, and to the square of the current 
which passes through it in a unit of time. Dr. Siemens pro- 
poses to take advantage of this well-established law of electro- 
dynamics, in order to limit and determine the amount of current 
passing through a circuit. The most essential part of the instru- 
ment employed for this purpose is a strip of copper, iron, or 
other metal, rolled extremely thin, through which the current to 
be regulated has to pass. One end of this thin strip of metal 
is attached to a screw, by which its tension can be regulated ; 
it then passes upwards over an elevated insulated pulley, and 
down again to the end of a short lever, working on an axis, 
armed with a counterweight and with a lever, whose angular 
position will be materially affected by any small elongation of 
the strip that may take place from any cause. The apparatus 
further consists of a number of prisms of metal, supported by 
means of metallic springs, so regulated by movable weights as 
to insure the equidistant position of each prism from its neigh- 
bour. The current passing through the thin strip of metal 
passes through the lever, and the line of prisms to the terminal, 
without encountering [any sensible resistance. A second and 
more circuitous route is, however, provided between the lever 
and the terminal, consisting of a series of comparatively thin 
coils of wire of German silver or other resisting metal, connect- 
ing the alternate ends of each two adjoining springs, the first 
and last spring being also connected to the lever and terminal 
respectively. Suppose that the current intended to be passed 
through the instrument is capable of maintaining the sensitive 
strip at a temperature of say 6o° C., and that a sudden increase 
of current takes place in consequence either of an augmentation 
of the supply of electricity or of a change in the extraneous 
resistance to be overcome, the result will be an augmentation of 
temperature, which will continue until a new equilibrium 
between the heat supplied and that lost by radiation is effected. 
If the strip is made of metal of high conductivity, such as 
copper or silver, and is rolled down to a thickness not exceeding 
0*05 millim., its capacity for heat is exceedingly small, and its 
surface being relatively very great, the new equilibrium between 
the supply of heat and its loss by radiation is effected almost 
