302 CARNEGIE INSTITUTION OF WASHINGTON. 



A rotary slide-wire for producing uniform variation in potential difference. S. J. Mauchly. 

 J. Opt. Soc. Amer. and Rev. Sci. Instr., vol. 6, No. 8, 852-858 (October, 1922). 



One of the requirements in the calibration of the recording conductivity- 

 apparatus in use at the atmospheric-electric observatories of the Department of 

 Terrestrial Magnetism is a device for varying the potential difference between 

 the two members of a condenser at a constant and definitely known rate.^ 

 The paper describes a special form of "rotary potentiometer" which was 

 devised for the use indicated and constructed in the instrument shop of the 

 Department. 



The slide-wire proper consists of about 25 meters of wire wound in a screw- 

 cut groove on a marble cylinder 10 cm. long and 10 cm. in diameter. The 

 cylinder is caused to turn on its axis by means of a small direct-current motor 

 which is energized by a storage battery. The traveling contactor consists 

 of a small grooved wheel which engages the spiral slide-wire and slides along 

 its own axis, which is parallel to the axis of the cylinder. The reduction gears 

 between the motor and cylinder and the other speed controls are such that 

 the time required for the contactor to travel the entire length of the cylinder 

 may be as short as 5 minutes and may be increased to 25 minutes. A tacho- 

 meter attached to the motor-shaft makes it possible to determine both the 

 speed of rotation of the cylinder and the degree of constancy at which this 

 speed is maintained. 



With the apparatus described it has been found possible to vary the poten- 

 tial difference between the traveling contact and either end of the slide-wire 

 at rates ranging from very small values up to about 0.1 volt per second and to 

 maintain constancy of rate to 1 part in 250. 



The paper is illustrated by 4 figures, which include 2 photographic views, 

 schematic diagram of connections, and a reproduction of drawings showing 

 the more important details of construction. 



The atmospheric-electric instrumental equipment for the observatories of the Department 

 of Terrestrial Magnetism. S. J. Mauchly. 



The program at the observatories of the Department of Terrestrial Mag- 

 netism at Watheroo, Western Australia, and at Huancayo, Peru, contem- 

 plates continuous records of the atmospheric-electric elements in addition to 

 those of terrestrial magnetism. The required electrographs so far designed 

 by the Department and constructed in its instrument-shop are for photo- 

 graphic registration of electric potential-gradient and conductivity of the 

 atmosphere. Designs are under way also for equipment to record photo- 

 graphically and continuously the ionization of the atmosphere. The essential 

 features of the potential-gradient and conductivity apparatus may be briefly 

 described as follows i^ 



The potential-gradient apparatus is essentially a quadrant electrometer 

 with needle connected to an insulated ionium collector. The collector is 

 supported 1 meter outside the wall of the atmospheric-electric house at a 

 height of 2.4 meters above the ground. The electrometer and the battery 

 connected to its quadrants are contained in a dust-proof and insect-proof 

 metal housing. The rod supporting the collector, together with its connection 

 to the electrometer is mounted in the axis of a tube of large diameter supported 



' See "An apparatus for automatically recording the electrical conductivity of the air," by 

 W. F. G. Swann, in "Annual report of the Director of the Department of Terrestrial Magnetism" 

 for the j-ear 1917, Year Book of the Carnegie Institution of Washington, 1917, p. 279. As indi- 

 cated in the paper referred to, a preliminary form of the apparatus here described was used by 

 Dr. Swann in 1917. 



* Complete descriptions of these instruments and of installations with illustrations will appear 

 n the Journal of Terrestrial Magnetism and Atmospheric Electricity. 



