REFORMS IN THE TEACHING OF ELECTRICITY 533 



tial by showing that there is a mechanical force of attraction 

 between bodies connected to points of a conductor which are at 

 different potentials, as he understands the term. To demon- 

 strate this, the voltmeter is connected to the mains and its 

 terminals are shunted with a high resistance of known value, 

 comparable with that already inserted in the live main ; he can 

 calculate by Ohm's Law the fraction of the electromotive force of 

 the source of supply that is employed in maintaining the current 

 in this shunt, so he sees that if a potential difference exist 

 between the fixed and movable plates of the voltmeter there is 

 an attraction between them and he can graduate the particular 

 instrument in use to measure potential differences in volts. 

 We do not yet go so far as to define potential in terms of work 

 or to calculate the relation which the force of attraction between 

 two bodies, of stated form and relative position, bears to their 

 difference of potential. But the pupil can now realise that it is 

 possible to give a mechanical definition of potential difference, 

 instead of the one he has used hitherto and that this latter then 

 becomes a derived unit, adopted for its practical convenience, 

 whose value may be determined by an absolute electrometer. 



We next demonstrate the varying capacities of bodies, by 

 showing how successive charges raise their potential and 

 by showing that a plate, first charged to the potential of the 

 mains when near an earthed plate, then insulated and removed 

 from the earthed plate, suffers a considerable rise in potential ; 

 this brings in the idea of a condenser. The student is already 

 acquainted with coulombs and he now learns the meaning of a 

 microfarad and is introduced to the use of condensers of a 

 commercial type. By experimentally sharing a charge between 

 a charged and uncharged condenser of stated capacities and by 

 simple reasoning, he reaches the equation Q == CV. If a con- 

 denser of 50 or 60 microfarad capacity be charged from 

 the mains through a megohm, the charging current continues 

 to flow during several minutes and if a microammeter be inserted 

 in each lead he can watch the changes in the charging current 

 and discover that equal currents flow through the two leads at 

 any instant ; hence he sees that there are equal quantities of 

 positive and negative electricity on the plates of a charged 

 condenser. 



We next use a condenser to show some of the phenomena of 

 induction. If both plates are insulated and one be raised to a 



