Relations between Electrical Measurements. 439 



we have to speak involves as factors measurements of time, space, 

 and mass only ; but these measurements enter sometimes at one 

 power, and sometimes at another. In passing from one set of 

 fundamental units to another, and for other purposes, it is 

 useful to know at what power each of these fundamental mea- 

 surements enters into the derived measure. 



Thus the value of a force is directly proportional to a length 

 and a mass, but inversely proportional to the square of a time. 

 This is expressed by saying that the dimensions of a force are 



-7™- ; in other words, if we wish to pass from the English to 



the French system of measurements, the French unit of force 



•ii * a ™ v-i 3-28x15-43 _ _ . . _ 



will be to the .Lnglisn as = : 1, or as ou'o to 1; 



because there are 3*28 feet in a metre, and 15 -43 grains in a 

 gramme. If the minute were chosen as the unit of time, the 



unit of force would, in either system, be ^ of that founded 



on the second as unit. 



A Table of the dimensions of every unit adopted in the present 

 treatise is given in § 55. 



Part II. — The Measurement of Magnetic Phenomena. 



5. Magnets and Magnetic Poles. — Certain natural bodies, as 

 the iron ore called loadstone, the earth itself, and pieces of steel 

 after being subjected to certain treatment, are found to possess 

 the following properties, and are called magnets. 



If one of these bodies be free to turn in any direction, the 

 presence of another will cause it to set itself in a position which 

 is conveniently described or defined by reference to certain ima- 

 ginary lines occupying a fixed position in the two bodies, and 

 called their magnetic axes. One object of our magnetic mea- 

 surements will be to determine the force which one magnet 

 exerts upon another. It is found by experiment that the 

 greatest manifestation of force exerted by one long thin magnet 

 on another occurs very near the ends of the two bars, and that 

 the two ends of any one long thin magnet possess opposite qua- 

 lities. This peculiarity has caused the name of u poles " to be 

 given to the ends of long magnets ; and this conception of a 

 magnet, as having two poles capable of exerting opposite forces 

 joined by a bar exerting no force, is so much the most familiar 

 that we shall not hesitate to employ it, especially as many of the 

 properties of magnets may be correctly expressed in this way ; 

 but it must be borne in mind, in speaking of poles, that they 

 do not really exist as points or centres of force at the ends of the 



