INSTRUMENTS, OBSERVATIONAL PROCEDURE, AND CONSTANTS 



the fiber deflections to obtain the value of the auxiliary 

 potential applied to the inner case. Record the fil)er 

 readings on a suitable form. (Department form No. 102) 



6. Commence observations of potential-gradient at 

 such a time that twenty readings taken one minute apart 

 will center on the average time of the observations of 

 conductivity, ion content, and nuclei content which are 

 being made simultaneously. Record the readings for the 

 twenty observations on the form. Between the tenth and 

 eleventh readings repeat item 5. 



7. After the twentieth observation repeat item 5. 



8. Record the position of mainsail and boom for the 

 period of observation. 



9. Record appropriate notes of weather conditions 

 for the period. 



10. Remove brass tube and umbrella; replace cover 

 over electrometer and axle. 



11. Calibrate the electrometer at least once each 

 week or more often if necessary, always with the inner 

 case earthed. With batteries brought on deck, apply po- 

 tentials in suitable steps to cover the desired range of 

 deflections on the scale of the electrometer. Record the 

 deflections and the battery voltages as determined jvith 

 a suitable voltmeter, on the calibration form. (Depart- 

 ment form 153a) 



It was necessary, of course, to determine the factor 

 by which the potential measurements should be multi- 

 plied to reduce them to values representative of the po- 

 tential-gradient over an open, flat surface undlstorted by 

 the presence of the ship. For the reduction factor de- 

 terminations a shore station was chosen at each of those 

 ports where there was comparatively flat open land, free 

 from trees and other objects, as nearly as possible at 

 the level of the surface of the sea, and not too distant 

 from the ship's anchorage (fig. 15). At the stations a 

 wire approximately twenty meters long, having sulphur 

 insulators attached at each end, was stretched horizon- 

 tally between two posts about one meter high. A metal 

 disc coated with ionium preparation was attached at the 

 mid-point of the wire and throughout the observations 

 the height of this "collector" was maintained as closely 

 as possible at one meter. The stretched wire was con- 

 nected at one end to an electrometer, located at a dis- 

 tance of two or three meters, by means of a fine wire, 

 and simultaneous eye readings were taken with this ap- 

 paratus and with the apparatus on the ship. Readings 

 isually were made at one-minute intervals for fifty 

 minutes of each hour, for a period of several hours. 

 During ten minutes of each hour, the collector was re- 

 moved from the stretched wire and the insulation of the 

 insulators supporting the stretched wire and of the elec- 

 trometer was tested. Bifilar electrometers 25 and 26 

 were used on different occasions for the shore observa- 

 tions. 



With potential-gradient recorders 4946 and 4947 

 available for reduction-factor determinations as well as 

 the eye-reading apparatus, continuous recording both on 

 the stern rail of the ship and at the shore stations could 

 be arranged. During cruise Vn five shore stations were 

 established and at each one some work was done with the 

 recording equipment, while a recorder was operating 

 also on the stern rail on all occasions except the first. 

 On two occasions continuous recording was maintained 

 both on ship and shore for several days. 



Positions of the mainsail and mainsail boom were 

 noted during reduction-factor observations as the boom 

 and sail extended astern far enough to contribute to dis- 

 tortion of the field. 



Careful note was made of weather conditions during 



these observations as it was known from previous work 

 that nearby clouds, variations in wind direction and ve- 

 locity, and changing conditions of mist or haze could 

 cause widely divergent values of potential-gradient at 

 the two locations, thus making the measurements ob- 

 tained in such circumstances unsuitable for determining 

 the reduction factor. 



Reduction factor determinations were made as fol- 

 lows: 



1. May 5, 1928 Kitts Point, Maryland, U. S. A. 



2. July 25, 1928 Reykjavik, Iceland 



3. Sep. 28-29, 1928 . . . Bridgetown, Barbados, B. W. I. 



4. Dec. 9-10, 1928. . . . Easter Island 



5. Apr. 10-13, 1929. . . Apia, Western Samoa 



The results obtained at these five stations were not con- 

 sistent with each other, and a study was made of the 

 data in an effort to determine the relative reliability of 

 the separate sets of material. A detailed report on this 

 study will be given later. Briefly, sets 1, 3, and 5 ap- 

 peared to have the greatest reliability, and on the basis 

 of these three the data from the other two were adjusted 

 to give compatible reduction factors. 



Potential-Gradient Recording Apparatus. --Cruise 

 VII was the first cruise of the Carnegie for which ar- 

 rangements were made for continuous recording of the 

 potential gradient. Initially it was planned that only eye- 

 reading apparatus 2 would be used at the stern of the 

 ship as on previous cruises, and that continuous record- 

 ing apparatus would be used at the top of the mainmast. 

 It was found, however, on the voyage from Norfolk, Vir- 

 ginia to Plymouth, England, May 10 to June 8, 1928, that 

 recording at the masthead was not practical. (See 

 progress reports, p. 31.) Recording on the roof of the 

 atmospheric-electric observatory was tried then and 

 also found to be impractical, so during the stay at Ham- 

 burg, Germany from June 22 to July 7, 1928, a platform 

 for the recorder was built on the stern rail of the ship to 

 starboard of the apparatus 2. On the platform was mount- 

 ed the metal-sheathed weatherproof wooden box contain- 

 ing the recording equipment, and from the top of the l)OX 

 projected a metal rod at the tip of which was mounted a 

 circular metal plate supporting, on its underside, four 

 small metal discs coated with the radioactive material 

 ionium. 



During experiments with the apparatus at the mast- 

 head, a short vertical rod was used to support the col- 

 lector plate. When the apparatus was installed on the 

 stern rail, a bent collector-rod which projected out over 

 the water directly astern was used at first. The collec- 

 tors at the end of the bent rod were approximately one 

 meter beyond the metal-sheathed Ixsx and in a horizontal 

 plane approximately 75 cm above the top of the box. This 

 arrangement is shown infigure 11, where the location of 

 the recorder with respect to apparatus 2 may be noted 

 also. 



The bent collector rod was used from July 7, 1928, 

 when the ship sailed from Hamburg, Germany, until No- 

 vember 5, 1929, eleven days after leaving Balboa, Canal 

 Zone. After leaving Balboa, bad weather was encoun- 

 tered and during the next eleven days the bent rod was 

 twisted out of place at frequent intervals and no useable 

 records of potential-gradient were obtained. A vertical 

 collector rod was installed on November 5, 1929, which 

 was not as short as that used in the masthead experi- 

 ments; it supported the collectors approximately 75 cm 



