ON STANDARDS OP ELECTRICAL RESISTANCE. 505 



to a negative cliarge ; aud the negative charge givca increased readings of 

 the micrometer, in the ordinary use of the instrument, to correspond to 

 positive charges of the principal electrode, as will be presently explained. 

 Before commencing to charge the jar, the upper attracting-plate should be 

 moved to nearly the highest position of its range by the micrometer-screw, 

 otherwise too strong a force of electric attraction may be put upon the 

 aluminium square ; and besides, the jar will discharge itself between the 

 upper plate and the extreme edge of the aluminium square, pulled as it is 

 very much above the level of the guard-plate by the electric attraction. I 

 have not found any injury or change of electric value of the scale-divisions 

 to aiise from any such rough usage ; but still, to guard against such a possi- 

 bility, I propose to add to the guard-plate cheeks to prevent the corners of 

 the aluminium from rising much, if at all, above its level, and to conduct the 

 discharge and protect the aluminiiim and platinum from the shock, in case of 

 the upper plate being brought too near the lower. When the instrument is 

 being charged, or when it is out of use at any time, the umbrella should 

 always be kept down ; but it must be raised to insulate the principal elec- 

 trode, of course, before proceeding to apply this to a body whose difference 

 of potential from a body connected with the case of the instrument is to be 

 measured. 



§ 30. In using the instrument the iimbrella must very frequently be lowered, 

 or metallic communication established in any other convenient way between 

 the chief electrode and the outer brass case, the micrometer-screw turned 

 until the hair takes its sighted position, and the reading taken, the hundreds 

 being read on the interior vertical scale, and the units (or single divisions of 

 the circle) on the graduated circle above. The number thus found is called 

 the earth-reading. It measures the distance from an arbitrary zero position 

 to the position in which the upper attracting-plate must be placed to give 

 the amount of electric force on the aluminium square which balances the 

 lever in its sighted position. A constant added to the earth-reading, or sub- 

 tracted from it, gives (§ 1) a number simply proportional to the difference 

 of potentials between the upper and lower plate ; that is to say, between the 

 two coatings of the Leyden jar. The vertical scale and micrometer-circle 

 are numbered, so that increased distances between the plates gives increased 

 readings ; and the zero reading should correspond as nearly as may be to 

 zero distance between them ; although in the instruments hitherto made 

 no pains have been taken to secure this condition, even somewhat approxi- 

 mately. If it is desired to know the constant, an electrical experiment 

 must be made to determine it, which is done with ease ; but this is not 

 necessary for the ordinary use of the instrument, which is as follows. 



§ 31. First, an earth-reading is taken, then the upper electrode is insulated 

 by raising the umbreUa, or otherwise breaking connexion between the prin- 

 cipal electrode and the outer metal case of the instrument. The principal 

 electrode and the outer case are then connected with the two bodies whose 

 difference of potential is to be determined, and the micrometer-screw is 

 turned until the hair is brought to its sighted position. The reading of 

 Inmdreds on the vertical scale and units on the circle is then taken. Lastly, 

 the principal electrode is again connected with the case of the instrument 

 and another earth-reading is taken. If the second earth-reading differs 

 from the first, the obser-ver must estimate the most probable earth-reading 

 for the moment when the hair was in its sighted position, with the upper 

 plate and the metal case in connexion with the two bodies whose difference 

 of potential is-to be measured. The estimated earth-reading is to be svib- 



