256 Proceedings of Societies . [March, 
instantaneously. But with the increase of temperature the posi- 
tion of the regulating lever is simultaneously affetfted, causing 
one or more contacfts to be liberated, and as many additional 
resistance coils to be thrown into circuit : the result being that 
the temperature of the strip varies only between very narrow 
limits, and that the current itself is rendered very uniform, notwith- 
standing considerable variation in its force, or in the resistance 
of the lamp, or other extraneous resistance which it is intended 
to regulate. The resistance coils, by which adjoining contact 
springs are connected, may be readily changed, so as to suit 
particular cases ; they are made by preference of naked wire, in 
order to expose the entire surface to the cooling adtion of the 
atmosphere. In dealing with feeble currents, Dr. Siemens uses 
another form of regulator, in which disks of carbon are substi- 
tuted for the wire rheostat. The eledtrical resistance of carbon 
varies inversely with the pressure to which it is subjected. A 
steel wire of say 0-3 millim. diameter is drawn tight between the 
end of a bell-crank lever, and an adjusting screw, the pressure of 
the lever being resisted by a pile of carbon disks placed in a 
vertical glass tube. The current passing through the steel wire, 
through the bell-crank lever, and through the carbon disks, 
encounters the minimum resistance in the latter so long as the 
tension of the wire is at its maximum ; whereas the least 
increase in temperature of the steel wire by the passage of the 
current causes a decrease of pressure upon the pile of carbon 
disks, and an increase in their eledtrical resistance. The instru- 
ment first described may be adapted also for the measurement 
of powerful eledtric currents. The variable rheostat is in this 
case dispensed with, and the lever carries at its end a pencil 
pressing with its point upon a strip of paper drawn under it in a 
parallel direction with the lever by means of clockwork. A 
second fixed pencil draws a second or datum line upon the strip, 
so adjusted that the lines drawn by the two pencils coincide 
when no current is passing through the sensitive strip. The 
passage of a current through the strip immediately causes the 
pencil attached to the lever to move away from the datum line, 
and the distance between the two lines represents the tempera- 
ture of the strip. This temperature depends, in the first place, 
upon the amount of current passing through the strip, and, in 
the second place, upon the loss of heat by radiation from the 
strip ; which two quantities balance one another during any 
interval that the current remains constant. The thin sensitive 
conductor may be utilised either to restrict the amount of elec- 
tricity flowing through a branch circuit, within certain narrow 
limits, or to produce a record of the amount of current passed 
through a circuit in any given time. 
January 30.- — “ A Comparison of the Variations of the 
Diurnal Range of Magnetic Declination as recorded at the 
