On the Intensity and Quantity of Electric Currents* 365 



In Ohm^s theory such suppositions are not required. 



According to this theory, two currents can only differ in one 

 respect, that is, in their strength; and this strength is measured 

 by their effect on the needle, and by their electrolytic action 

 during a unit of time. 



The strength of the current in Ohm^s theory is also called 

 intensity, and it is supposed to be proportional to the quantity 

 of electricity which passes through any cross section of the cir- 

 cuit during the unit of time. 



This meaning of the word intensity, and the previous use of 

 the same word, must be carefully distinguished. Ohm^s so-called 

 intensity agrees rather with the meaning of the term quantity, 

 as above used, than with that of intensity. 



According to Ohm, the intensity or strength of a current 

 depends upon two circumstances. First, it is directly propor- 

 tional to the electromotive force which gives rise to the current, 

 and it is indifferent whether this arises from chemical action or 

 from any other cause; secondly, it is inversely proportional to 

 the resistance in the whole circuit, this resistance being the sum 

 of the resistances in all parts of the circuit*. Hence the greater 

 the resistance of the circuit already is, the less will the intensity 

 of the current be diminished by any given increase of the 

 resistance. 



By means of this simple view it is easy to explain all thephse- 

 nomena for which the hypothesis of different proportions in the 

 intensity and quantity of the currents was assumed. To make 

 this more easily understood, it is convenient to divide the resist- 

 ance of every circuit into two parts. 



One part of the resistance belongs to that part of the circuit 

 in which the current is generated, that is, within the battery 

 itself. As this part of the circuit is indispensably necessary to 

 the production of the current, the corresponding part of the 

 resistance may therefore be called the internal or necessary 

 resistance. The other part of the resistance belongs to that part 

 of the circuit which is not essential to the production of the cui*- 

 rent, and which is generally considered as acted upon by the 

 current ; as, for example, the galvanometer wire or the volta- 

 meter which may be introduced into the circuit of the above- 

 mentioned battery. This part of the resistance may be called 

 the external or accidental resistance. 



; By comparing the necessary resistance in different kinds of 

 circuits, it may be easily ascertained that all those currents to 

 which a considerable intensity in proportion to the quantity has 

 been attributed, arise from sources of great necessary resistance. 

 Indeed in iin^e electrical machine the necessary resistance is almost 

 Y«^i-trrejjp oi* See Phil. Mag. S. 4. vol. iii. p. 324. 



