ON STANDARDS OF ELECTRICAL RESISTANCE. 501 



the amount of weight to be placed on the upper side of the disk to bring the 

 hair to its sighted position when there is no electric force is determined. 

 This last condition is secured by putting the two plates in metallic commu- 

 nication with one another. For the electric experiments the weight is 

 removed, so that when the hair is in the sighted position the electric attraction 

 on the moveable disk is equal to the force of gravity on the weight. The 

 electric connexions suitable in using this instrument for determining in 

 absolute electrostatic measure the difference of potentials maintained by 

 a galvanic battery between its two electrodes are indicated in fig. 11. 

 'No details as to the case for preventing disturbance by em-rents of aii-, and 

 for maintaining a diy atmosphere, by aid of pumice impregnated mth strong 

 sulphm-ic acid, are shown, because they are by no means convenient in the 

 instrument at present in use, which has undergone so many transformations 

 that scarcely any part of the original structure remains. I hope soon to 

 constract a compact instrument convenient for general use. The amount of 

 force which is constant in each series of experiments may be varied from one 

 series to another by changing the position of a small wire rider on the lever 

 from which the moveable disk is liung. 



The electric system here described is heterostatic (§ 40 below), there being 

 an independent electrification besides that whose difference of potential is to 

 be measured. 



POETABLE ElECTKOMETER. 



§ 23. In the ordinary use of the portable electrometer (figs. 8, 9, & 10, 

 Plate VI.), the electric system is heterostatic and quite similar to that of the 

 absolute electrometer, when used in the manner described above in § 22. 

 But the balance is not adapted for absolute measure of the amount of force 

 of attraction experienced by the moveable disk ; on the contrary, it is pre- 

 cisely the same as that described for the gauge of the quadrant electrometer 

 in § 13 above, only tui'ucd upside down. Thus, iu the portable instrument, 

 the square disk (/) forming part of the lever of thin sheet aluminium is 

 attracted upwards by a sohd circular disk of sheet-brass {y), thick enough 

 for stiffness. Every part of the aluminium lever except this square portion 

 is protected from electric attraction by a fixed brass plate Qih) with a square 

 hole in it, as nearly as may be stopped by the square part of the sheet 

 aluminium destined to experience the electric attraction, all other parts of 

 the aluminium balance-lever being below this g-uard-plate. The aluminium 

 lever {i Jc), as shown in figs. 8 & 10, is shaped so that Avhen the hair (I) at 

 the long end of its lever is in its sighted position, the upper sm-faces of the 

 fixed guard-plate (h) and moveable aluminium square (_/') are as nearly as 

 may be in one plane. The mode of siLspension is precisely the same as that 

 described (§ 13) for the gauge of the quackaut electrometer. In the portable 

 instrument, careful attention is given by the maker to balance the aluminium 

 lever by adding to it small masses of shellac or other convenient substance, 

 so that its centre of gravity may be in the line of its piatinum-wii-e axis, 

 or, more properly speaking, in such a position that the instrument shall give, 

 when electrified, the same " earth-readings " when held in any positions, 

 cither upright, or inchned, or inverted (§ 30 below). Thus the condition of 

 equilibrium of the balance, when the hair is in its sighted position, i» that 

 the moment of electric attraction round the axis of suspension shall be equal 

 to the moment of the couple of torsion, the latter being as constant as the 

 properties of the matter concerned (platinum wire, brass stretching-springs, 

 &c.) will allow. 



