ELECTRICAL UNITS OF MEASUREMENT. 93 



a convenient unit ; but the common mode of 

 measuring force by reference to weight without 

 reference to locality is not definite, because the 

 weight of a gramme is different here from what it 

 is at the Equator. The heaviness of a pound or 

 a gramme is greater by a two-hundredth at either 

 pole than at the Equator ; or to give the exact 

 figures, 0.00512. That is a difference of \ per 

 cent, and if your accuracy is to be within a \ 

 per cent., you cannot ignore the difference of the 

 force of gravity in different places. But a vast 

 number of measurements in engineering, and in 

 the most ultra scientific work of scientific labora- 

 tories, does not aspire to so high a degree 01 

 accuracy ; and for all such work the local or 

 terrestrial-gravitation unit suffices, without speci- 

 fying what the particular place is only that it 

 is somewhere or other on the face of the earth. 

 For instance, moduluses of rigidity, moduluses of 

 rupture, breaking strains of material, are stated 

 accurately enough for engineering purposes, in 

 terms of a ton weight per square centimetre, or 

 grammes weight per square centimetre, or any 



