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The first unit of electric measurement that we must consider 
is that of resistance, as well because it is at once the most 
important one, and it is the one from which all the others are 
derived, either directly or indirectly. 
George Simon Ohm discovered about the year 1827, that when 
electricity travelled over a wire, the wire offered a certain amount 
of resistance to its passage; he further found that if a thick wire 
was used this resistance was less, and also the same was the case 
when the wire was short. Stated shortly, ‘the resistance 
offered to the passage of electricity over and through it is 
inversely in proportion to its thickness and its length.” A thick 
short wire will offer a less resistance than a short thin wire. 
This element of electrical resistance is one of great importance, 
not only theoretically but practically, and it was recognised very 
early after electricity became of commercial importance, that its 
measurement must be provided for. Not only must this unit be 
ascertained, but it is self-evident that it should be ascertained as 
accurately as possible. Many workers in this department of 
science, more especially the Siemens Brothers, the Cavendish 
Laboratory of Cambridge, Sir William Thomson, and a Com- 
mittee of the British Association carefully worked upon it, and 
after many years of painstaking labour arrived at a definite 
conclusion. This unit of resistance, it was decided to call one 
Ohm ; but it was decided to found it strictly upon the metrical 
system. ‘‘ A proposal had been before the world for many years 
to found this unit of electric resistance upon a material of 
uniform quality, and one at the same time easily obtained, and 
which by easy precautions could be obtained in a state of perfect 
purity.” 
The Siemens’ unit was therefore proposed, and this unit a 
column of mercury one metre in length and one square milli- 
metre, ‘03987 of an inch in section. 
The great house of Siemens—Sir William in England, and Dr. 
Werner Siemens in Germany—worked in a most thorough and 
powerful way to give us a unit of electric resistance, the measure 
of resistances, in terms of the specific resistance of mercury, in 
such a manner as to give us a standard, easily reproduced in any 
place, with no other measure at hand than the metre measure. 
This unit—though not professing to be an absolute unit at all, 
but only a convenient one—was accepted as the foundation for 
the formation of an absolute unit. 
The system of Gauss consists simply in defining the unit of 
force, which, acting upon a unit of mass, generates a velocity 
equal to unit of velocity. 
