364 Reports on Special Researches 



OBSERVATIONS ON CRUISE III OF THE GALILEE, 1907-1908. 



(W. J. Peters in Command.) 



The observations included measurements of the potential-gradient, conductivity, and 

 the radioactive deposit on a charged wire. They were made on the cruise of the Galilee 

 from Sitka (Alaska) to Honolulu (Hawaiian Islands), Marshall Islands, Lyttelton (New Zea- 

 land), Callao (Peru), and San Francisco, during the period August 12, 1907, to May 15, 

 1908. The observer was P. H. Dike, who had been sent to Europe in 1906 by the Depart- 

 ment of Terrestrial Magnetism to receive special training, for the proposed atmospheric- 

 electric work, at Berlin and Potsdam, at Wolfenbiittel (under Professors Elster and Geitel) 

 and at the University of Cambridge. The following extracts are taken from his report.' 



The determination of the potential-gradient, after careful consideration of the condi- 

 tions on board a saiUng vessel, seemed quite impracticable, and no serious attempt was made 

 to secure observations. The rolUng of the ship, the flapping of the sails, and the varying 

 positions of the yards and boom under various sailing conditions all contributed to make 

 the problem of reducing observations of potential-gradient to a unifonn basis too compli- 

 cated to be undertaken in the initial work. On board a steamer the conditions would be 

 less variable and it might be possible to reduce readings to values for undisturbed sea by 

 means of simultaneous observations in port with the vessel at anchor and the second 

 collector and electroscope mounted on a raft at some distance from the vessel. 



It was possible only once to secure potential observations at sea, viz, on December 7, 

 1907, during a period of absolute calm, when even the long swell had almost died out, in 

 latitude 22° 40' south and longitude 170° 36' east. A small skiff was put overboard, and 

 the writer, assisted by Observer D. C. Sowers, rowed out about 100 yards from the ship. 

 The Elster-and-Geitel flame collector was set up on its ebonite rod, at a height above sea- 

 level estimated at 2 meters. Large, and extremely variable, potentials were obtained, 

 varying rapidly from zero to potentials beyond the range of the electroscope. The mean 

 value would be not far from 90 volts per meter. The conditions on tliis day were so 

 abnormal that not much value can be assigned to the observations, though they encouraged 

 the assumption that the potential-gradient over the sea is not so very different from that 

 over the land. 



It was hoped to investigate the amount of radioactive material in the atmosphere 

 by Elster and Geitel's method. In this method a wire of definite length is charged to a 

 potential of —2,500 volts and exposed to the atmosphere for a period of two hours, after 

 which it is quickly coiled on a frame and introduced into an ionization chamber connected 

 to an electroscope. If the ionization chamber and electroscope are of the proper capacity 

 and dimensions, the activity is said to be unity when the initial fall in volts per hour per 

 meter of wire introduced is unity. Owing, however, to breakage in the box of dry piles 

 in transportation and the consequent failure of the means of maintaining a high potential 

 on the charged wire, the radioactivity work was not satisfactory, as it was not found 

 possible to reach a potential much above 1,000 volts, even with the box of dry piles opened 

 to the hot sun. However, several exposures of a copper wire about 10 meters long were 

 made dvu-mg the first half of the voyage. In the neighborhood of land, as off the coast of 

 Alaska and in Cook Strait, New Zealand, December 21, 1907, the observations showed 

 conclusively the presence of radioactive emanation in the air, even with the low potential 

 available for charging the wire. In Cook Strait a value for A (the " Aktivierungszahl " of 

 Elster and Geitel) of 40 was found, the deposit decaying to half value in about 40 miiiutes. 

 But in the open sea no increase in the rate of discharge of the electroscope used for testing 

 the exposed wire could be detected. With a better charging device it might be possible to 

 obtain some result, but it would probably be only a small fraction of that on or near land. 



■Sep Tnr. Mag., vol. 13, pp. 119-128, 1908. 



