438 Prof. Maxwell and Mr. F. Jenkin on the Elementary 



The phenomena by which electricity is known to us are of a 

 mechanical kind, and therefore they must be measured by mecha- 

 nical units or standards. Our task is to explain how these units 

 may be derived from the elementary ones ; in other words, we 

 shall endeavour to show how all electric phenomena may be 

 measured in terms of time, mass, and space only, referring 

 briefly in each case to a practical method of effecting the 

 observation. 



3. Standard Mechanical Units. — In this country the standard 

 of length is one yard, but a foot is the unit popularly adopted. 

 In France it is the ten millionth part of the distance from the 

 pole to the equator, measured along the earth's surface, accord- 

 ing to the calculations of Delambre ; and this measure is called a 

 metre, and is equal to 3-280899 feet, or 39*37079 inches. 



The standard unit of time in all civilized countries is deduced 

 from the time of rotation of the earth about its axis. The side- 

 real day, or the true period of rotation of the earth, can be ascer- 

 tained with great exactness by the ordinary observations of astro- 

 nomers ; and the mean solar day can be deduced from this by 

 our knowledge of the length of the year. The unit of time 

 adopted in all physical researches is one second of mean solar 

 time. 



The standard unit of mass is in this country the avoirdupois 

 pound, as we received it from our ancestors. The grain is one 

 7000th of a pound. In the French system it is the gramme, 

 derived from the unit of length by the use of water at a stan- 

 dard temperature as a standard of density. One cubic centi- 

 metre of water is a gramme =15*43235 grains ='00220462 lb. 



A Table showing the relative value of the standard and derived 

 units in the British and metrical systems is given in § 55. 



The unit of force adopted in this treatise is that force which 

 will produce a unit of velocity in a free unit mass, by acting on 

 it during a unit of time. This unit of force is equal to the 

 weight of the unit mass divided by g, where g is the accele- 

 rating force of gravity 



= 32-088 (1 + 0-005133 sin 2 X) in Brit, units') at the level 

 or =9-78024(1 + 0005133 sin 2 X) in met. units/ of the sea, 



\ being the latitude of the place of observation. A unit of force 

 still very generally adopted is the weight of the standard mass. 



The value of the new unit is - times the old or gravitation 



unit. 



The unit of work adopted in this treatise is the unit of force, 

 defined as above, acting through the unit of space (vide § 55). 



4. Dimensions of Derived Units. — Every measurement of which 



