62 
In short, then, the resistance unit is such that a current 
requiring one second of time to travel over one quadrant of the 
earth’s circumference, shall have a resistance of one ohm to 
oppose and overcome. 
This distance being approximately one billion centimetres, a 
current travelling with the velocity of one billion centimetres per 
second, is one ohm. 
We now come to another unit. 
The Volt, called after Alvessandro Volta, a Professor of Padua. 
If we take a Daniell’s Cell, such as this one, and cause a 
current to flow in it through the wire connecting the two metals 
of which it is composed, copper and zinc—namely, from the 
copper to the zine; as water flows always in the direction of 
least resistance, so does the electric current, and this direction of 
flow is due to an analogous cause—that is, it flows from the 
higher to the lower point. 
Now the difference of the intensity of the current at the copper 
pole from that at the zine pole, is called ‘ difference of 
potential,” and it flows from the metal having the higher 
potential to the metal having the lower one, from the copper to 
the zinc. 
The volt, then, is the difference of potential between the poles 
of a Daniell’s cell; rather it is a little less than this, for these 
nine cells should theoretically have a potential of nearly ten volts. 
Defining a volt in terms of velocity per second, we define the 
ohm ; it is a current flowing between the poles with a velocity of 
one hundred million centimetres per second. The enormous 
rapidity of the motion of electricity is well known. 
We have now got the unit of resistance, the ohm; and the 
unit of current, the volt. 
(Ampére). We have yet to get the unit of quantity; this can 
be got in two ways. If a current flows through a wire of one 
ohm resistance with one volt of potential, we have it, and it is 
called the ampére, after another of the pioneers in electrical 
science. But we can get it in a more accurate way by estimating 
the work done by a given current in one second. This work may 
be the decomposition of water or the electrical depositions of 
metals. It has been found by experiment carefully carried out, 
over and over again, that a current decomposing 0°00009362 
grammes of water in one second, is of the strength of one 
ampére, and that one ampére of current will decompose so much 
water per second of time, or deposit 0:00111815 grammes or 
0:017258 grains of silver per second. This is the chemical 
definition of the ampére, and is at the same time a more or less 
