Early Atmospheric Electricity Measurement Technology
Early measurements of the atmospheric potential over land used an insulated electrode exposed to the atmosphere above the surface, connected to a mechanical electrometer to allow the extent of the electrification to be found (Harrison 2004a; Nicoll 2012). The sensing electrode or “collector” used to acquire the air’s local electric potential was initially typically a flame or elevated point (Harrison 2004a). Electrometers of the eighteenth and nineteenth centuries employed electrostatic repulsion, such as with pith balls (Read 1792) or the deflection of straws or fibres (Hackmann 1994).
In 1860, Lord Kelvin described a “water dropper” collector system (Everett 1868), which consisted of an insulated tank continuously generating a spray of water. At the point where the water broke into drops, the spray transferred charge into the air until its potential became equal with the local potential of air. This was found by measuring the potential of the tank that was measured with respect to Earth, and a continuous chart record could be obtained photographically. Although in principle this provided an absolute method for determining the air potential at the spray generation point (and, therefore, the vertical potential gradient between this point and Earth), Kelvin was aware that the effect of an electrode could be to distort the local field being measured (Thomson 1859). To determine the geometrical correction, measurements without field distortion can be obtained by using a plate electrode, mounted flush with, but insulated from, level ground. Such an implementation was ultimately used at the Kew Observatory site near London (Harrison 2003; Harrison and Ingram 2005) with a procedure devised by WilsonFootnote 8 (1906), near to the Kelvin water dropper recording system.
Balloon-carried measurements of atmospheric electricity parameters began in the late nineteenth century and also employed water dropper technology (Harrison and Bennett 2007; Nicoll 2012). For the measurement of air conductivity,Footnote 9 an aspirated tubular sensor was developed (Gerdien 1905). This consisted of a well-insulated central electrode, mounted concentrically with an outer tubular electrode (see Nicoll 2012, Figure 5). After charging the central electrode to a known potential, air was drawn through the tube by a fan, and the rate of decay of the central electrode voltage measured. This allowed the air conductivity to be found (e.g., Aplin 2005). An additional feature of the “Gerdien tube” was that its outer electrode provided some electrostatic and physical shielding of its sensitive inner electrode.
The first PG measurements aboard a ship at sea seem likely to have been those of Exner, made between Aden and Bombay in 1888 (Simpson and Wright 1911). Clearly, the approach of using a flush-mounted plate electrode over undistorted ground was unsuitable for use on board ship. However, the need to correct for the field distortion was, if anything, even more acute than over land, as, for sailing ships, almost every location on board was strongly influenced by the presence of yards, sails and ropes, whose positions constantly changed. Realising this, in the pioneering marine PG observations of Simpson and Wright (Simpson and Wright 1911) on the Terra Nova during 1910, measurements made on board the ship were calibrated to additional measurements determined over a level beach. This site was 1,000 yards from the ship, where a long (18 m) fine horizontal wire, well insulated at each end, had been arranged. Because of its very small cross-section, and the substantial distance between the mounting posts, the long fine wire “antenna” provided minimal field distortion and, therefore, reference PG values.
The ratio of the ship-measured PG to the beach-measured PG provided the geometrical correction for field distortion, known as the reduction factor, which, once determined, could be applied to the ship PG measurements if the sail and rigging geometry remained unchanged. (The reduction factor remains a fundamental consideration for all PG measurements, in which placement of a sensor distorts the local field; see, for example, Bennett and Harrison 2006.)
One further aspect of the Terra Nova and the earlier measurements of SimpsonFootnote 10 in Lapland (Simpson 1906) was that the collector used was coated with radioactive material. This enhanced the local ionisation and, therefore, the air’s local conductivity, increasing the rate at which the collector acquired the air potential, giving better time resolution.
Origin of the Carnegie Oceanic Atmospheric Electricity Measurements
Inspiration to include atmospheric electricity amongst the Carnegie Institution’s measurements can be traced back to the German scientists J. Elster and H. Geitel,Footnote 11 when plans for the ocean magnetic surveys were being developed. As magnetic and atmospheric electricity measurements had long often been combined, such as at Kew Observatory in the UK (Ronalds 1847; Harrison 2003), Elster and Geitel may even have expected that atmospheric electricity measurements would be made alongside the magnetic observations. They made their aspiration for the inclusion of atmospheric electricity measurements clear in a letterFootnote 12 to the Carnegie Institution written on 26 January 1902:
…we beg leave to suggest that it would be in full harmony with the proposed plan to combine with the organization of international magnetic work also the inauguration of observations pertaining to the electric condition of the Earth and of the atmosphere…
In response, the Carnegie Institution Director, Dr. L. Bauer,Footnote 13 visited Europe in spring 1903, meeting, as well as Elster and Geitel, other key researchers in atmospheric science (notably von Bezold, Chree, Ebert, Mascart, Schuster, Shaw, Rucker and Wiechert). Initially, it was not thought wise to embark on ocean atmospheric electricity work until the basic problems of accurate oceanic magnetic work were solved, so preliminary atmospheric measurements were delayed until the third and final cruise of the Galilee. These did not prove at all easy; in fact, it was reportedFootnote 14 that PG measurements
…seemed quite impracticable…the rolling of the ship, the flapping of the sails, and the varying position of the yards and boom under various sailing conditions all contributed to make the problem of reducing observations of potential-gradient to a uniform basis too complicated…
Of several atmospheric electricity instruments originally considered for evaluation on the Galilee, only air conductivity measurements, using the Gerdien aspirated device (Gerdien 1905), apparently showed any promise for ocean measurements. Air ion measurements continued to be made on Cruise I of the Carnegie (1909–1910), alongside observations of “specific conductivity” and “radioactive content”.
On the Carnegie’s second cruise (1910–1913), as well as continuing with conductivity and radioactivity measurements, PG measurements were attempted again. Radioactive collectors were used with a mechanical electrometer, with the collectors suspended on a bamboo pole extending back from the ship’s stern rail. A similar method was used on Carnegie Cruise III (8 June 1914 to 21 October 1914, between Brooklyn, Hammerfest, Reykjavik and Brooklyn), supporting an ioniumFootnote 15 collector from the rear of the ship. By making simultaneous ship and shore observations, at Reykjavik and Gardiners Bay, a correction for the electrostatic distortion due to the ship’s structure, known as the reduction factor, was obtained. This allowed the PG measurements for the whole of Cruise III to be calibrated, giving the average for the cruise as 93 Vm−1.
Cruises IV, V and VI
During Cruises IV (March 1915 to March 1917), V (December 1917 to June 1918) and VI (October 1919 to November 1921), improvements in the instruments and methods were made steadily, with increasing attention to obtaining diurnal variation results. An “atmospheric-electric house” was built aboard for Cruise IV, to protect the instruments and provide stable operating conditions, but it also minimised the setting-up time for measurements. However, the PG instrumentation, using the ionium-coated collector extended on a bamboo pole, was found increasingly unsatisfactory for regular measurements, as the collector required at least 2 min for the electrometer to reach 1 V of its final steady potential. This led to a fundamentally new design of collector, employing a sensing surface reminiscent of an open parasol (Fig. 2). This collector was made from wire gauze, arranged on a horizontally projecting pivot arm. In use, it was rotated from pointing downwards (“zero”) to a horizontal measurement position, during which the associated change in electrometer reading was recorded. The operating position could be adjusted for the tilt of the ship. A further advantage of measuring just the change in the electrometer reading was that the insulation requirements were reduced, with a sulphur main insulator entirely satisfactory, as long as the hard rubber insulation for the handle was kept clean with fine emery cloth. Even so, each morning the observer tested the apparatus for leakage with a 100 V Zamboni pile. It was also found that the ship’s rail made it unnecessary to use a wire screen to eliminate inductive effects associated with movements of the observers.
Standardisation of the measurements on the ship to open surface measurements was achieved by using simultaneous observations at sea and on land. These comparison sites were chosen to be close to level with the sea and free from trees, with a horizontal passive wire antenna of 15–20 m long employed as the reference measurement. Satisfactory sites were hard to find; however, as there were difficulties in maintaining the ship’s mooring within half a mile, the reduction factor was originally measured only for short periods (minutes) and averaged. Later, these standardisation experiments were extended to durations of a few hours, with the linearity between the two sets of measurements demonstrated before deriving the reduction factor. A further complication was the distortion of the electric field by changes in the arrangement of the ship’s sails. The PG measurements were only made when the mainsail was up with the boom to port or starboard, or when mainsail down and the boom positioned “some 2 feet over the port crutch”. These differences were quantitatively important, as apparent from the range of final valuesFootnote 16 of the reduction factor used for 1915–1921, which varied from 2.85 (mainsail up and boom to port or starboard) to 3.77 (mainsail down and boom 2 feet over port crutch). Because changes in reduction factor affected the final values derived by the same proportional amount, they were allowed for in the results by recording the state of the sails and boom.
By the end of Cruise V, the parasol collector had become badly corroded, so the apparatus was entirely rebuilt, also implementing a stronger support rod. This arrangement (“PG2”) was made to similar dimensions to keep the reduction factor the same. Reduction factors for PG2 were first made in Colon Harbor (at the entry to the Panama Canal) on 2 April 1915, and applied retrospectively to the data from the first year of Cruise IV. Further tests after Cruise VI on nonlinearities in the reduction factor found no variation, and it was said to be “practically constant”, for PG from 120 to 480 Vm−1.
By 1921, analysing the relative variations from the previous decade’s cruises, it was very clear that the chief contribution to the diurnal variation of PG was the 24 h component, and, further, that, as reported by its discoverer, the Carnegie Institution’s scientist S. J. Mauchly,Footnote 17
…the 24 hour Fourier wave was at the great majority of land stations in practical phase agreement on universal time with the prime daily wave over the oceans without regard to location.Footnote 18