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On the scale value of the Delhnan Electro meter used 

 hji Dr. A. Wislizenus. 



By Francis E. Nipher. 



In the Transactions of this Academy, vol. ii., pp. 115, 3S7, 

 414, 526, Dr. Wishzenus gives a table of observations on at- 

 mospheric electricity with a Dellman electrometer. In 1S75 the 

 writer visited Dr. Wislizenus, and, as he was then unable to 

 make further observations with the instrument, he consented to 

 deposit it in the physical cabinet of Washington University. 



The series of observations made by him was the first, and for 

 years tiie only series made in this country, and, in order that the 

 potentials measured by him my be known in volts, the writer has 

 made a comparison of the instrument with a Thomson electro- 

 static voltmeter. The source of electricity was a Rowland water 

 battery consisting of four plates, having in all 2,500 cells. 



The battery was first examined to determine whether any of the 

 cells were short-circuited. This was done by cutting out the cells 

 fifty at a time, and reading the potentials on the Thomson instru- 

 ment. This determination was repeated at various times and the 

 battery was found to give very constant results, so that the poten- 

 tials could always be determined by simply determining the num- 

 ber of cells in circuit. This was the more necessary as the Thom- 

 son instrument does not give reliable values below 500 volts. 



The Dellman instrument is so constructed that when the nee- 

 dle is at zero it stands directly over the bar which is to repel it. 

 The needle cannot therefore be brought to zero by turning the 

 torsion-head, as the needle when charged would then be in an 

 unstable condition. The arrangement of the torsion-head is such 

 that it was evidently not intended to be used in this manner. 

 The measurements were made by first reducing the whole instru- 

 ment to zero potertial, adjusting the needle to zero by means of 

 the torsion-head, and then charging the deflection-bar and needle, 

 and reading the deflection of the needle. This was the method 

 used by Dr. Wislizenus. 



The needle was found to be slightly bent laterally, so that the 

 ends could not be brought to zero when the centre of the needle 



