496 REPORT— 1867. 



along with the amount of torsion in the platinum ■wire bearing the aluminium 

 halance-arm, so as to give the proper sensibility to the gauge. The sensibility 

 is increased by diminishing the distance from the attracting to the attracted 

 plate, and increasing the amount of torsion. Or, again, the degree of the 

 potential indicated by it when the hair is in the sighted position is increased 

 by increasing the distance between the plates, or by diminishing the amount 

 of torsion. If the electrification of the needle is too great, its proper position 

 of equilibrium becomes unstable ; or before this there is sometimes a liability 

 to discbarge by a spark across some of the air-spaces. The instrument works 

 extremely weU with the needle charged but little less than to give rise to one 

 or both of these faults, and I adjust the gauge accordingly. 



§ l-t. The strength of the fixed steel-directing magnets is to be adjusted to 

 give the desired amount of deflection with any stated difi'erence of potentials 

 maintained between the two chief electrodes, when the jar is charged to the 

 degree which brings the hair of the gauge to its sighted position. In the 

 instruments already made, the deflection* by a single ceU of Daniell's amounts 

 to about 100 scale-divisions (of jL of an inch each at a distance of 40 inches), 

 when the magnetic directive force is such 'as to give a period of vibration 

 equal to about 1'5 secoud. When the jar is discharged and the four 

 quadrants are connected with one another and witli the inner coating of the 

 jar, lower degrees of sensibiHty may be attained better by increasing the 

 magnetic directing-force than by diminishing the charge of the jar. Thus, 

 for instance, when it is to be used for measuring and photograiibically re- 

 cording the potential of atmospheric electricity at the point where the stream 

 of the water-dropping collector t breaks into di'ops, the magnetic directing- 

 force maj'^ be made from 10 to 100. times more than that just described. 

 When this is to be done it may be convenient to attach a somewhat more 

 powerful magnetic needle than that which has been made in the most recent 

 instruments where a high degree of sensibility is desired. Eut it is to be re- 

 marked that in general the directing-force of the external steel magnets 

 cannot be too sti'ong, as the stronger it is the less is the disturbance produced 

 by changing magnetic bodies in the neighbourhood of the instrument. In 

 laboratory work, where numerous magnetic experiments are being performed 

 in the immediate neighbourhood, and in telegraph factories where there is 

 constant disturbance by large moving masses of iron, the artificial magnetic 

 field of the electrometer ought to be made very strong. To allow this, and 

 yet leave suificient sensibility to the instrument, the suspended magnetic 

 needle has been made smaller and smaller, until it is now reduced to two 

 small pieces of steel side by side, 6 millimetres (^ of an inch) long. For a 

 meteorological observatorj^ all that is necessary is, that the directing magnetic 

 force should be so great that the greatest disturbance experienced in mag- 

 netic storms shall not sensibly deflect the luminous image J. 



§ 15. The sensibility of the gauge should be so adjusted that a variation 

 in the charge of the jar, producing an easily perceived change in the position 

 of the hair, shall produce no sensible change in the deflection of the luminous 

 image produced by the greatest difference of potentials between the qua- 

 drants, which is to be measured in the use of the instrument. I beheve the 



* That is to say, the number of scale-divisions over which the himinous image moves 

 when the chief electrodes are disconnected from one another and put in metallic connexion 

 with the two plates of a Daniell's battery. 



t See R(3yal Institution Lecture, ]\Iay 18, ISGO (Proceedings of tlie R. I.), or Nieliol's 

 Cyclopnedia, article " Electricity, Atmospheric." (Edition 1860.) 



I All embarrassment from this soiu'ce will be dono away with if the bifilar plan be 

 adopted {vide footnote to § 8). 



