136 REPORT—18693. 
Part ITIl.—Merssvrement oF Exrecrric PHENOMENA BY THEIR ELECTRO- 
MAGNETIC EFFECTS. 
13. Preliminary.—Before treating of electrical measurements, the exact 
meaning in which the words “ quantity,” ‘‘ current,” ‘ electromotive force,’ 
and “ resistance” are used will be explained. But, in giving these explana- 
tions, we shall assume the reader to be acquainted with the meaning of such 
expressions as conductor, insulator, voltaic battery, &e. 
14, Meaning of the words “ Electric Quantity.””—When two light conduct- 
ing bodies are connected with the same pole of a voltaic battery, while the 
other pole is connected with the earth, they may be observed to repel one 
another. The two poles produce equal and similar effects. When the two 
bodies are connected with opposite poles, they attract one another. Bodies, 
when in a condition to exert this peculiar force one on the other, are said to 
be electrified, or charged with electricity. These words are mere names given 
to a peculiar condition of matter. Ifa piece of glass and a piece of resin are 
rubbed together, the glass will be found to be in the same condition as an 
insulated body connected with the copper pole of the battery, and the resin 
in the same condition as the body connected with the zine pole of the 
battery. The former is said to be positively, and the latter negatively 
electrified. The propriety of this antithesis will soon appear. The force with 
which one electrified body acts on another, even at a constant distance, varies 
with different circumstances. When the force between the two bodies at a 
constant distance, and separated by air, is observed to increase, it is said to 
be due to an increase in the quantity of electricity ; and the quantity at any 
spot is defined as proportional to the force with which it acts, through air, on 
some other constant quantity at a distance. If two bodies, charged each 
with a given quantity of electricity, are incorporated, the single body thus 
composed will be charged with the sum of the two quantities. It is this fact 
which justifies the use of the word “ quantity.” 
Thus the quality in virtue of which a body exerts the peculiar force 
described is called electricity, and its quantity is measured (ceteris paribus) 
by measuring force. 
The quantity thus defined produced on two similar balls similarly circum- 
stanced, but connected with opposite poles of a voltaic battery, is equal, but 
opposite ; so that the sum of these two equal and opposite quantities is zero ; 
hence the conception of positive and negative quantities. 
In speaking of a quantity of electricity, we need not conceive it as a sepa- 
rate thing, or entity distinct from ponderable matter, any more than in 
speaking of sound we conceive it as having a distinct existence. Still it is 
convenient to speak of the intensity or velocity of sound, to avoid tedious 
cireumlocution ; and quite similarly we may speak of electricity, without for 
a moment imagining that any real electric fluid exists. 
The laws according to which the force described varies, as the shape of the 
conductors, their combinations, and their distances are varied, have been 
established by Coulomb, Poisson, Green, W. Thomson, and others. These 
will be found accurately described, independently of all hypothesis, in papers 
by Professor W. Thomson, published in the Cambridge Mathematical Journal, 
vol. i. p. 75 (1846), and a series of papers in 1848 and 1849. 
15. Meaning of the words “ Electric Current.”—When two balls charged by 
the opposite poles of a battery, with opposite and equal quantities of elec- 
tricity, are joined by a conductor, they lose in a very short time their pecu- 
liar properties, and assume a neutral condition intermediate between the 
