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MR. WHEATSTONE ON NEW INSTRUMENTS AND PROCESSES FOR 
of the principal results which have been deduced from it. It will soon be perceived 
how the clear ideas of electro-motive forces and resistances, substituted for the vague 
notions of intensity and quantity which have been so long prevalent, enable us to 
give satisfactory explanations of most important phenomena, the laws of which have 
hitherto been involved in obscurity and doubt. Viewing the laws of the electric 
circuit from the point at which the labours of Ohm has placed us, there is scarcely 
any branch of experimental science in which so many and such various phenomena 
are expressed by formulae of such simplicity and generality ; in most of the physical 
sciences the facts of observation and experiment have kept pace with theoretical ge- 
neralization, in this science alone they had gone on accumulating in prolific abundance 
without any successful attempt having been made to reduce them to mathematical ex- 
pression. But this is now happily effected, and what has hitherto been mere matter 
of speculative conjecture is removed into the domain of positive philosophy. 
By electro-motive force is meant the cause which in a closed circuit originates an 
electric current, or in an unclosed one gives rise to an electroscopic tension. By re- 
sistance is signified the obstacle opposed to the passage of the electric current by the 
bodies through which it has to pass ; it is the inverse of what is usually called their 
conducting power. 
When the activity of any portion of the circuit is increased or diminished, either by 
a change in the electro-motive force or in the resistance of that portion, the activity 
of all the other parts of the circuit increases or decreases in a corresponding degree, 
so that the same quantity of electricity always passes in the same instant of time 
through every transverse section of the circuit. 
The force of the current is directly proportional to the sum of the electro-motive 
forces which are active in the circuit, and inversely proportional to the total resist- 
ance of all its parts, or in other words the force of the current is equal to the sum of 
the electro-motive forces divided by the sum of the resistances. 
Let F denote the force of the current, E the electro-motive forces, and R the resist- 
ances : then 
The length of a copper wire of a given thickness, the resistance of which is equi- 
valent to the sum of the resistances in a circuit. Ohm calls its reduced length, an ex- 
pression which it will frequently be found convenient to employ. 
If the electro-motive forces and resistances in a circuit are proportionately increased 
or diminished the force of the current remains the same, or Hence a single 
voltaic element, or a battery consisting of any number of exactly similar elements, if 
no additional resistance be interposed in the circuit, produces the same effect. Also 
a thermo-electric element and a voltaic element will produce the same effect when 
the greatly inferior electro-motive force of the former is compensated by a corre- 
