Atmospheric-Electric Observations, 1915-16 381 



Another disadvantage attending the use of any form of collector shows itself from a 

 consideration of the fact that since the electric force near the stern of a ship is a function 

 not only of the electric force in the open, but of the configoiration of that surface which is 

 bounded by the sea and the stern of the ship, the force must be a quantity which fluctuates 

 continually as that configvu-ation changes, due to the rolling and pitching of the vessel. 

 In order that potential-gradient measurements shall have a definite meaning, it is therefore 

 desirable that they shall always be taken at one position of the tilt of the ship. 



The apparatus devised for observations on the Carnegie was designed with a view to 

 overcome the above objections. A picture of it is shown in Plate 22, Figure 4, and the 

 principle of its action will be clear from Figure 20. The instioiment is a modification of an 

 earUer one^ designed for preliminary use on the third cruise of the Carnegie. It comprises 

 a brass tube A fixed at one end to an axle so that it can rotate in a plane containing the 

 fore-and-aft line of the ship. The axle is mounted on supports fixed to the stern rail of the 

 ship, and the projecting end of the brass tube carries a gauze disk B made somewhat in the 

 form of a parasol. The handle C by which the rotation is brought about is insulated 

 from the axle, and the latter is itself insulated from Earth by causing it to work in brass 

 tubes fixed into their supports with 

 sulphur insulation. The axle is con- 

 nected by a thin wire to a Wulf bifilar 

 electroscope D, the wire and axle being 

 in the same line. It is arranged that 

 when the brass tube is approximately 

 vertical and the parasol attachment 

 downward, the electroscope system is 

 earthed . On rotating the tube to some 

 other position fixed by a stop, a deflec- 

 tion proportional to the potential-gra- 

 dient is obtained in the electroscope. 

 Insulation difficulties are entirely over- 

 come, since the leak occurring during 

 the turning of the handle from one 

 position to another is negligible ; fur- 

 ther, the operation can be performed so quickly that a reading can be obtained at any 

 desired position of tilt of the ship. The sensitivity is considerable, and it is easy to 

 arrange so that, for the nonnal value of the potential-gradient, deflections amounting to 

 the whole scale length are obtained. 



Although the principle of the apparatus is such that a high degree of insulation is not 

 essential, the sulphur supports for the axle were provided with caps holding attachments for 

 drying material. It has, however, not been found necessary to use any drying material; 

 and indeed, during the whole period in which the apparatus has been in use on Cruise 

 IV, hardly any occasion has been recorded in which insulation trouble was experienced 

 with this instrument, although observations were frequently taken under conditions of 

 great dampness. The arrangements are such that the parasol attachment can be fixed in 

 position or removed in a moment, and this is the only part of the apparatus which is taken 

 away when the observations are completed. The axle and electroscope system remain in 

 position and are covered with a suitable cover. Two leads from a battery stored in a room 

 some distance away come up to the base of the instrument, and enable a potential to be 

 apphed to the subsidiary case of the Wulf electroscope, so that the range of the instrument 

 may be adjusted to suit special conditions. This arrangement also enables the sign of the 

 potential-gradient to be determined by noting the direction of movement of the deflected 

 fibers when a small potential of known sign is applied to the subsidiary case. 



'See W. F. G. Swann, Terr. Mag., vol. 19. pp. 182-185, 1914. 



Fig. 20. — Diagram of Potential-Gradient Apparatus. 



