ON ohm's law. 57 



found, however, that with some care the break could be got to work long 

 enough to allow of good results being obtained. 



On account of this gradual alteration of the break, and for other reasons 

 n-i well, it was of vital importance to be able during the experiment to obtain 

 some measure of the amount of current that passed as representative of the 

 large and small current respectively ; for the experiment would obviously be 

 nugatory if, instead of the smaller current being nearly half the larger, 

 it became, owing to deterioration of contact in the cup S, equal to what ought 

 to be the larger current. To provide for this the experiments were conducted 

 as follows : — The balance was found, whether for larger currents alone or 

 smaller alone (acting directly or with the fork going), or for both together 

 in the same or in opposite directions ; then the block was moved as quickly 

 as possible 6 centims. from the position of balance, and the deflection which 

 then appeared was read off; this deflection is approximately proportional to 

 the current. Knowing then the electromotive force of either battery and its 

 internal resistance, one could not only tell v.'hether the currents wore passing 

 nearly in the right proportion, but also estimate roughly how much current 

 absolutely passed in each case. In some of the best experiments a more ac- 

 curate method was adopted: — The point D was "put to earth," and the point 

 E connected by means of a long insulated wire with one pair of the quadrants 

 of a Thomson's electrometer in the flat of the laboratory below the room where 

 tlic experiment was carried on; the other pair of quadrants being "put to 

 earth," the deflection observed on the electrometer-scale was a direct measure 

 of the electromotive force between D and E — that is, of the quantity denoted 



above by g^E. 



Before giving the quantitative results obtained from the most satisfactory 

 experiments, it maybe well to explain the principle on which these have been 

 selected from the others. In all the experiments quoted there was either 

 something remarkable, such as a high battery power, &c., or else the balances 

 were obtained imder very favourable circumstances, the spot of light being 

 very steady, and the proportions of current passing, as indicated by the 

 sensibilities* or electrometer measurements, being near the theoretically best 

 amounts. Often where the breaks were not working satisfactorily, by work- 

 ing quickly a qualitative experiment could be made, the behaviour of the 

 galvanometer indicating to an observer practised in the experiment that the 

 ])roportions of current passing were not far wrong; and often part of an 

 experiment could be made perfectly satisfactorily, and then the apparatus 

 Avould go out of order. But in all the experiments, whenever the results were 

 at all intelligible (regular), the conclusion pointed to never differed from that 

 given by the best experiments, viz. either the balance for the currents in 

 opposite direction lay more to the left than that for the currents in the same 

 direction, or the two coincided. Of this the observer spared no trouble in 

 assuring himself even in experiments that wero quantitatively utterly value- 

 less. 



The first set of experiments quoted, which are not of much value quan- 

 titatively, may serve to illustrate what has just been said. In this set the 

 time is given because the experiments 'were made during the slow heating- 

 effect already alluded to. The spot of light was not perfectly steadj-, though 



much steadier for the -\ balances than for the others ; the bridge-reading 



is given to tenths of a millimetre, though of course in the present case for 



* The deflection clue to six centimetres deviation from balance is called the sensibility. 



