506 REPOiix — 1867. 



tracted from the readiug taken in connexion with the hodies to he tested. 

 This diflPerence measures (§ 21) the required difference of potentials between 

 them iu units of the instrument. The yalue of the unit of the instrument 

 ought to be known in absolute electrostatic measure ; and the difference of 

 reading found in any experiment is to be multiplied by this, which is called 

 (§1) the absolute coefficient of the instrument, to give the required dif- 

 ference of potentials in absolute measure. It so happens that, in the 

 portable electrometers cf the kind now" described which have been hitherto 

 constructed, the absolute coefficient is somewhere about -01, so that one turn 

 of the screw, or 100 divisions of the circle, corresponds to somewhere 

 about one electrostatic unit, with a gramme for the unit of mass, a centi- 

 metre for the unit of distance, and a second for the unit of time ; but the 

 different instruments differ from one another by as much as ten or twenty 

 per cent, in their absolute coefficients. In all of these I have fouud between 

 three and four Daniell's cells to correspond to the unit division ; that is to 

 say, between three hundred and four hundred cells to a full turn of the 

 screw. With great care, the observer may measure small differences of 

 potentials by this instrument to the tenth part of a division (or to about 

 half a Daniell's cell). With a very moderate amount of practice ard care, 

 an error of as much as a half division may be avoided in each reading. 



§ 32. But there are imperfections in the instrument itself which make it 

 difficult or impossible to secure very minute accuracy, especially in measure- 

 ments throiigh wide ranges. 



(1) In the first place, I am not sure that the end of the needle 

 carrj-ing the hair is protected sufficiently by the wire fences (§ 25) from 

 electric disturbance to provide against any error from this source, which 

 possibly introduces serious irregularities. 



(2) In the second place, the capacity of the jar in the small portable 

 instrument is not sufficient to secure that the potential of its inner coating 

 shall not differ sensibly with the different distances to which the upper plate 

 is brought, to balance the aluminium lever with the hair in its sighted po- 

 sition. But on this point it is to be remarked that the electric density on 

 the upper surface of the guard-plate is in its central parts always the same 

 when the hair is in its sighted position ; and it is therefore only the compa- 

 ratively small diflPerence of the quantity of electricity on this surface, towards 

 the rim, corresponding to different distances of the attracted plate, that 

 causes diflPerence of potential in the inner coating of the jar. But if the upper 

 attracting-plate be kept for several minutes at any distance, diftering by a 

 few turns of the screw, from that which brings the hair to its sighted 

 position, the electricity creeps along the inner unconnected surface of the 

 glass so as to increase the charge of the inner metallic coating, or diminish 

 it, according as the distance is too great or too small. If then quickly the 

 screw be turned and the earth-reading taken, it is found greater or smaller, 

 as the case may be, than previously ; but after a few minutes more it returns 

 to its previous value very approximately. Error from this source may be 

 practically avoided by taking care never to allow the hair to remain for more 

 than a few minutes far from its sighted position ; never so far, for instance, 

 as above the centre of the upper, or below the centre of the lower dots. 



(3) A third source of error arises from change of temperature influencing 

 the indications. In most of the instruments hitherto made 1 have found 

 that the warmth of the hand produces in a few minutes a very notable aug- 

 mentation of the earth-reading (as it were an increased charge in the jar) ; 

 but in the last instrument which I have tested (White No, 18) I find the 



