ELECTRICAL STANDARDS. 
BY KAEL E. GUTHE, 
As soon as any commodity becomes of commercial value a system of 
practical units has t@ be established for its measurement. In the case 
of electrical energy we have fortunately been saved from the natural 
development of a system of units, a development which has, for exam- 
ple, given us our remarkably complex, incongruous, and unscientific col- 
lection of weights and measures. While by no means perfect the elec- 
trical units have at least the advantage of being consistent, and being 
worked out by scientists they have naturally been based upon the metric 
system. I do not wish to tire you with a lengthy description of the var- 
ious methods which have been employed for the derivation of the dif- 
ferent electrical, or to be more specific, electro-magnetic units, in terms of 
the fundam«ntal units, the centimeter, gram, and second, — suffice it to 
state that of the three most important electrical units two, that of the 
resistance, the Ohm, and that of current, the Ampere, can be determined 
by so-called absolute methods, in which determinations only length, mass 
and time enter into the measurements, while the third, the unit of elec- 
tromotive force, the Volt, is found from the other two by the use of 
Ohm’s law. 
All other electrical quantities can easily be derived from these three 
by the application of well-known mathematical relations. 
As can be imagined determinations of the absolute or c. g. s. units 
are difficult and require a great deal of labor and time; so it is natural 
that an attempt was made to substitute for these theoretical units prac- 
tical and wherever possible, concrete and easily reproducible units, 
standing in definite relation to the former. This is not difficult in the 
case of the unit of resistance, the Ohm. A column of mercury of defin- 
ite mass and length at a specified temperature will always have the same 
electrical resistance to a constant current and by a series of classical 
experiments* it was determined some years ago that the Ohm which 
equals 10"'* c. g. s. units of resistance is represented with sufficient 
accuracy by the resistance, at the temperature of melting ice, of a 
column of mercury, 14.4521 grammes in mass, of a constant cross sectional 
area and of the length of 106.3 centimeters. 
At the International Electrical Congress at Chicago, in 1893, this 
unit was adopted and thus replaced some of the older measures which did 
not as closely correspond to the theoretical value. For a perfectly satis- 
factory definition of the mercury ohm, it is, however, necessary to add 
to the already given data the method of filling the tube, the form of the 
=i=Dorn, Zeit. f. Instr.— Kunde, 1893, APP. 
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