UNITS AND STANDARDS OF MEASUREMENT. 379 



equivalent of heat, we know that the assumed unit of 

 heat is equal to the energy of 423*55 gramme -metres, 

 or that energy which will raise the mass of 423*55 

 grammes through one metre against 9*80868 absolute 

 units of force. Heat may also be expressed in terms of 

 the quantity of ice at o Cent., which it is capable of con- 

 verting into water under an inappreciable pressure. 



The science of electricity has lately become so much a 

 matter of quantity, that it is necessary to have some 

 means of accurate expression. When we know exactly 

 the mechanical equivalent of electricity, we can express 

 quantities of electricity in terms of energy, but in the 

 meantime we need some easy available unit. The British 

 Association accordingly have selected as the unit of 

 electrical force that which can just overcome the resistance 

 offered by a piece of pure silver wire i metre in length, 

 and i millemetre in diameter. This unit must be re- 

 garded as merely a convenient provision for working 

 purposes, to be employed for the easy expression of 

 quantities not yet brought into precise relation with the 

 ultimate standards of time, space, and mass. There may 

 also be other provisionally independent units employed 

 in electrical science, such as the voltametric unit of cur- 

 rent strength, namely, that current which by decomposing 

 water produces one cubic centimetre of detonating gas at 

 o Cent, and 7 60 mm. of pressure in one minute. The unit 

 of electrical quantity, again, is that quantity which when 

 concentrated in a point and acting on an equal quantity 

 also concentrated in a point at a unit of distance, exerts 

 a repulsion equal to the unit of force. There must also 

 be a unit of electro-magnetic force. All these electrical 

 units must, however, be definitely related to each other, 

 and to the fundamental units, and it is a matter for 

 continual investigation to determine such relations more 

 and more accurately. 



