452 PROCEEDINGS OF THE AMERICAN ACADEMY 



eliminate errors of parallax in reading the position of a fine alumininin 

 index playing over it. This uppermost comi)artraent contains the sus- 

 pension apparatus and the long light aluminium index arm. The sus- 

 jjension is as previously described. Tlic aluminium pointer is carried 

 by the platinum. wire which supports the needle, but is insulated from 

 it. A small concave mirror is also attached to the platinum wire, so 

 that, if desired, the instrument can be employed with lamp and scale 

 as a reflecting instrument. In the middle compartment IJ, four brass 

 quadrants are mounted on flint-glass tubes of 4 cm. in length and 1 cm. 

 in diameter. Each quadrant is compounded of four single quadrants. 

 The dividing partitions fit into slots cut iu the back plate, and are re- 

 movable at pleasure. They are held in an exact horizontal position 

 by means of small screws. The needle is made of aluminium, and is 

 also of a compound type, being made of four or more single needles, 

 connected and so arranged as to move between the quadrant sections. 

 The interior surface of compartment B is completely tinfoiled. The 

 third compartment contains a glass jar tinfoiled on the outer side and 

 in connection with the ground. Through the side of the glass vessel 

 is led a platinum wire encased in hard rubber. The deflection of the 

 needle is recorded by the movement of the aluminium pointer. In 

 the instrument constructed, when one set of quadrants is connected 

 with the ground, the other to the positive pole of a Daniell cell, and 

 the needle connected with the positive pole of a Beetz battery of 200 

 cells (described below), the movement of the index hand is percepti- 

 ble to the unaided eye. On a scale distant 70 cm. from the mirror 

 this deflection is nearly 2 cm. The length of the suspension is about 

 4 cm. Increasing the potential of the needle increases the sensibility 

 of the instrument. If, instead of the method generally employed, 

 we connect one set of quadrants with the positive pole of a battery 

 of a number of cells connected in series, and the negative pole to 

 the other set of quadrants and the needle connected with the body 

 whose potential is to be determined, we obtain greater sensitiveness. 

 The deflection obtained has then to be compared directly with the 

 deflection given by a Daniell cell. 



Connected iu this manner, our electrometer gave a movement of 

 the index hand, for a Daniell cell, of several degrees, or, with the 

 mirror and scale, a deflection of about 4 cm. 



For getting a continuous record, this form of electrometer is more 

 easily adaptable than the others. It is also obvious that, aside from 

 the dilficulty and uncertainty of photography, an electrometer for 

 successful use in meteorological work must be of such a nature 



