28 M. W. Siemens on a new reproducible Standard 
or oP Gad 
w- nro 1+/a+ /a 
or ¥ 1 
watt ptiala 
G 3 
When R and are nearly equal, the correction for the conical 
form of the conductor, viz. the factor 
differs little from unity ; whence every tube not accurately cylin. 
drical may be regarded as a truncated pyramid without netice- 
able error, the ratio a bemg determined from the greatest and 
least lengths of the mercury thread employed in gauging its 
capacity. 
By a series of experiments I now ascertained whether the cal- 
culated amount of the resistances of a number of tubes of very 
different mean sections agreed sufficiently well with the results 
of measurement. The method I pursued was as follows :— 
Glass tubes, from about 3 to 2 millims. internal diameter, were 
fastened to a long graduated scale, drops of mercury were intro- 
duced into them, and the length of the thread of mercury in the 
tube was measured. By inclining the tube under examination, 
the thread of mercury was made to pass gradually down its entire 
length; and thus every piece of tube of about 1 metre long 
which appeared most nearly cylindrical or uniformly conical, was 
detected. These pieces were then cut out of the tube, and, by 
means of a sinall apparatus constructed for that purpose by M. 
Halske, were reduced to the exact length of 1 metre. The tubes 
so prepared were then carefully cleaned. This was most conve- 
niently done by twisting together two thin German silver or steel 
silk-covered wires, inserting them through the tube, twisting a 
pad of clean cotton to the projecting end, and drawing it slowly 
and cautiously through the tube. This operation required to be 
performed with some care to avoid, breaking the tube. The tube 
was then filled with clean mercury and its contents weighed. This 
operation was conducted as follows :—One end of the glass tube 
was fastened, by means of a piece of vulcanized caoutchoue, to the 
opening of a small retort receiver, such as is used in chemical 
laboratories, so that the end of the tube projected ito the 
receiver. About the other end of the tube an iron collar was 
attached (fig. 2), by means of which a smooth iron plate could 
