129 
were intended to be used merely in a technical sense. By the 
intensity magnet I designated a piece of soft iron so surrounded 
with wire that its magnetic power could be called into operation by 
an ‘intensity’ battery, and by a guwant:ty magnet a piece of iron 
so surrounded by a number of separate coils that its magnetism 
could be fully developed by a ‘quantity’ battery.’’* Clearly, 
therefore, we owe to Henry the credit of having first worked out 
practically the functions of two entirely different kinds of electro- 
magnets ; one having several separate coils of no great length, desig- 
nated as a ‘‘ quantity ’’ magnet, the other provided with a continu- 
ous coil of very considerable length, designated as an ‘‘ intensity ’’ 
magnet. ‘‘ The latter and feebler system (requiring for its action 
a battery of numerous elements) was shown to have the singular 
capability (never before suspected or imagined) of subtle excitation 
from a distant source. Here for the first time is experimentally 
established the important principle that there must be a proportion 
between the aggregate internal resistance of the battery and the 
whole external resistance of the conjunctive wire or conducting 
circuit ; with the very important practical consequence that by com- 
bining with an ‘intensity’ magnet of a single extended fine coil 
an ‘intensity’ battery of many small pairs, its electro-motive 
force enables a very long conductor to be employed without diminu- 
tion of the effect. This was a very important though unconscious 
experimental confirmation of the mathematical theory of Ohm, 
embodied in his formula expressing the relation between electric 
flow and electric resistance, which, though propounded two or three 
years previously, failed for a long time to attract any attention 
from the scientific world.’’+ 
The practical outcome of these experiments was a most important 
one. Although Ampére, at the suggestion of Laplace, had exam- 
ined the question and had shown the possibility of making a tele- 
graph by deflecting a needle through a long length of conducting 
wire, yet further experiments by Barlow proved that lengthening 
the conducting wire did actually produce a diminution of the effect. 
Even with only two hundred feet of wire he found such a sensible 
diminution as to convince him of the impracticability of the scheme. 
From Henry’s experiment just described, however, ‘‘ it appears that 
* Smithsonian Report for 1857, p. 108. 
+ W. B. Taylor’s address, Memorial of Joseph Henry, published by order of Congress, 
Washington, 1880, p. 227. 
PROC. AMER. PHILOS. SOC. XXXII. 143. Q. PRINTED DEC. 18, 1893. 
