APPENDIX. 



QUADRANT KLECTHOMETRY WITH A FREE LIGHT NEEDLE, HIGHLY CHARGED THROUGH A 



CONDUCTOR OF IONIZED AIR.' 



1. The principles of construction aimed at in tlie following paper are: (1) 

 To have the needle swing fi'eely in aii-, /. e., without liquid contacts ; To devise 

 means for suitably charging this needle (2) by induction alone; (3) By transmis- 

 sion of the charge to be measured (potential) by ionized aii- ; (4) Practically, to 

 compensate for the loss of charge of the highly chai'ged needle by conduction 

 thi'ough ionized air. 



To my knowledge tlie sensitiveness of Kelvin's original apparatus built upon 

 a Leyden jar has remained unrivalled. Realizing the ingenious use which is made 

 of sulphuric acid wliich in the instrument is a desiccator, a damper, a conductor, 

 and pai't of a receptacle for storing lai'ge charges, it has nevertheless seemed to me 

 that a useful modification might be attainable if the liquid contact of the needle 

 could be obviated. In relation to such small toi'ques as are successfully applied in 

 galvanometry, it appears that the large capillaiy and viscous resistances introduced 

 by partial submergence in a liquid, must ultimately, or in other dispositions, prove 

 objectionable. Nobody would, for instance, think of damping an ordinaiy galvano- 

 meter needle in a liquid. It has, tlierefore, long been my purpose to reduce the 

 torsional conditions of the needle to quuitz fiber accuracy. 



There is a second point of view: Notwithstanding the sensitiveness of 

 Kelvin's beautiful instruments, they have not found as much favor in general 

 practice as the modification of them devised by Maseait. The troublesome high 

 potentials of Kelvin's instrument are here i-eplaced by a water battery, with 

 advantage and without serious sacrifice, as the sensitiveness of the original instru- 

 ment was, as a rule, excessive. Mascart, however, retained the liquid damper. If 

 this can also be removed, I think that a markedly increased sensitiveness must be 

 attainable with all the conveniences due to Mascart's modification. Incidentally it 

 appears that inasmuch as the liipiid contact means a platinum vane, etc., the heavy 

 parts are needless in a free needle, a furthei- favorable condition. 



In the apparatus presently to be described, I have succeeded in installing an 

 air damped light, aperiodic, free needle, and have in this way increased the seusi- 



' Cf. Meeting Nat. Acad. Sciences, Philadelphia, November, 1901. Since that day, an instru- 

 ment made with similar ends in view, but on a different plan, has been described by F. Dolezalek. 

 Zeits.f. Instrum'k., xxi., pp. 345-35°, 'QO'- The capsule attached to my instrument is the feature 

 which I wish to accentuate. 



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