280 



ELEMENTARY LESSONS ON [CHAP. v. 



Seeing, however, that quantities a million times as great as 

 some of these, and a million times as small as some, have to be 

 measured by electricians, the prefixes mega- and micro- are 

 sometimes used to signify respectively " one million " and " one- 

 millionth part." Thus a megohm is a resistance of one million 

 ohms, a microfarad a capacity of 1 oo * OOQ of a farad, etc. 

 The prefix milli- is frequently used for "one-thousandth part;" 

 thus a milKweber is the thousandth part of one weber. 



This system of "practical" units was devised by a committee 

 of the British Association, who also determined the value of the 

 "ohm" (or " B. A. unit," as it is sometimes called) by experi- 

 ment, and constructed standard resistance coils of metallic wires. 

 The "practical" system may be regarded as a system of units 

 derived not from the fundamental units of centimetre, gramme, 

 and second, but from a system in which, while the unit of time 

 remains the second, the units of length and mass are respectively 

 the earth -quadrant and 10 n gramme. 



324. Dimensions of Magnetic and Electromagnetic Units. 

 The fundamental idea of "dimensions" is explained in Art. 

 258. A little consideration will enable the student to deduce 

 for himself the following table 



UNITS. 



DlMENSl 



(Magnetic. ) 



( Strength of pole 



( Quantity of magnetism 



Magnetic Potential 

 Intensity of Field 



( Electro-magnetic. } 

 Current (strength) 

 Quantity 



Potential 

 Electromotive-Force 



Resistance 

 Capacity 



= A/force X (distance^ 



= work -f- strength of pole 

 force -f- strength of pole 



intensity of field x length 

 current x time 



work -f- quantity 



E.M.F. -f- current 

 quantity -f- potential 



M'2 L* "j 

 Ma L 



LT1 



