ELECTRICAL MEASUREMENTS. 279 



the corresponding term in the case of a current of anything 

 else. If we are speaking of a current of water, we mean 

 by the strength of a current the quantity of water 

 delivered in a given time, say a minute. Similarly 

 the quantity of electricity conveyed along a conductor 

 by a given current in a second or minute is the strength 

 of the current. Yery frequently, however, processes are 

 spoken of as measurements of the strength of currents 

 which really enable us merely to compare one current with 

 another. They are comparative measures, but not absolute 

 measures. We might have several water pipes delivering 

 water each at its own rate, and we might possibly ascertain 

 that one delivers three times as much per minute as another, 

 and another one sixteen times as much as the first, without 

 knowing what is the absolute quantity delivered by either 

 of them, and we have in electrical, as in all other measure- 

 ments, to distinguish between such comparative measure- 

 ments and absolute measurements which tell us, not only 

 whether a thing is greater or less than some other thing 

 of the same kind, but actually how great it is. And it is 

 the general principles of the methods by which absolute 

 measurements of the strength of currents can be obtained 

 that I shall have to speak of. 



There are three chief properties or three effects pro- 

 duced by electric currents, any one of which might be 

 made the basis of a system of measurement. You all 

 know that when an electric current passes near a magnetic 

 needle it exerts force upon the needle, and usually dis- 

 places the needle from its ordinary position; a current 

 passing near a magnet deflects it, so that we may say that 

 an electric current produces electro magnetic effects ; 

 and we may use the term electromagnetic action of a 

 current to include all the mutual effects which take place 

 between currents and magnets. Upon this action of 

 electric currents then we might found a system of absolute 

 measurement. Then, again, one electric current passing 

 near another electric current exerts a force upon it, or 

 rather when two currents are near each other there is in 

 general a mutual force tending to displace them relatively 

 to each other. The forces exerted by electric currents 

 upon each other, usually spoken of as electro-dynamic 

 forces, might serve as the basis of a second system of 



