March 24, 192 1] 



NATURE 



T19 



An important paper on the "corona voltmeter " was 

 read to the American Institute of Electrical Engineers 

 last July by Prof. J. B. Whitehead, of Johns Hopkins 

 University. The principle on which the voltmeter is 

 founded is that a corona forms on a clean, round 

 wire in air at a sharply marked definite value of the 

 voltage dependent only on the pressure and tem- 

 perature of the air. The voltage at which the corona 

 forms can be observed directly by the eye or by the 

 •deflection of a galvanometer in the high-tension cir- 

 cuit, or best by the sound made in a telephone. The 

 wire on which the corona forms is in a chamber the 

 pressure of the air in which can be varied. This instru- 

 ment gives a higher accuracy than that obtainable 

 by a sphere-gap voltmeter, and the presence of neigh- 

 bouring conductors does not affect its readings. An 

 instrument on this principle to read 100,000 volts can 

 easily be constructed in any electrical laboratory. 

 The author is making one to read 400,000 volts. In 

 experimenting with these voltmeters on alternating 

 pressures a curious physical law was discovered. If 

 R denotes the maximum potential gradient in 

 kilovolts per cm., and r the radius of the wire in 

 cm., then at 25° C. and 76 cm. pressure the value of 

 R at which the corona appears is given by 

 R = 29-84+9-938/v'r, provided that i/v^r is less 

 than 226; but if the value of i/V'r is greater 

 than 226, R = 3296+ 8-559/ \/r. The reason given as 



an explanation of this sudden change in the law is 

 that the laws governing the formation of the positive 

 and negative coronas found with direct voltages are 

 slightly different. 



In addition to a large amount of useful statistical 

 information in a paper on fuel oil read by Mr. W. A. 

 White before the North-East Coast Institution of En- 

 gineers and Shipbuilders on January 28, there is a 

 section in which the advantages of fuel oil over coal 

 are enumerated for power purposes at sea. Fuel oil 

 lends itself more easily to complete combustion than 

 any solid fuel ; owing to the higher heating value 

 there is a saving in dead-weight, and increased space 

 may be devoted to cargo ; the conditions governing 

 the speed of the ship are better, and there is economy 

 regarding the necessary crew. In relation to the last- 

 mentioned point, the Aquitama while burning coal had 

 a staff of 350 men in connection with the stokeholds, 

 and now on fuel oil this vessel requires some 84 men 

 only. For bunkering the Aquitania has four fuel- 

 receiving lines, and 480 tons per hour have been pumped 

 into her bunkers from one barge through one pipe- 

 line ; the total quantity of fuel required for her round 

 trip could easily be delivered into the bunkers in six 

 hours. Before conversion to oil-burning the Aquitania 

 and the Olympic each took about 108 hours at each 

 end for coal-bunkering, and employed 50 to 60 men ; 

 oil-bunkering employs 3 men only. 



Our Astronomical Column. 



New Comet 1921a. — A comet of the 9th magni- 

 tude was discovered by Mr. Reid at the Cape Observa- 

 tory on March 13. The following observations have 

 been received : 



S. Decl. 



Place 



G.M.T. R.A. 



d. h. m, h. in. s. o / // 



March 14 14 51 o 20 14 35*0 18 28 48 Johannesburg 



,, 18 16 497 20 16 567 16 20 40 Algiers 



Deduced daily motion +35S., N. 31^'. Predicted 

 place March 25d. i6h., R.A. 2oh. 21m., S. declination 

 12° 41'. The comet will rise on that day about 2|h. 

 before sunrise. It cannot be identical with comet 

 Pons-Winnecke, for the latter passes its ascending 

 node near aphelion, whereas the new comet passed 

 its ascending node about noon on March 12. 



Careful search has been made for Pons-Winnecke 

 by several observers without success. Either the 

 comet is unexpectedly faint or it is a long way from 

 the predicted place. 



Re-appearance of Saturn's Ring. — The Comptes 

 rendus of the Paris Academy of Sciences for 

 February 28 contains the observations of this pheno- 

 menon made at Strasbourg by MM. A. Danjon and G. 

 Rougier. The smaller equatorial (aperture 16 cm.) 

 was emploj'ed. From February 11 to 21 no trace 

 of the ring was visible outside the disc ; its shadow 

 on the disc appeared as a black line 0-2" broad. 



On February 22, at gh. G.M.T. , the ring was seen 

 as a very narrow bright line with condensations dis- 

 tant i8-2' and 13-8" from the centre of the disc, being 

 strongest on the eastern side. These measures, and 

 the others in the article, are reduced to the mean 

 distance of Saturn from the sun. The visibility of 

 the ring increased perceptibly during the four hours of 

 observation, and on the following night it was quite 

 an easy object. Making use of Barnard's measures 

 of the ring system, the authors show that the con- 

 densations measured by them were respectively a 



NO. 2682, VOL. 107] 



little inside the middle of ring A, and about one-fourth 

 of the width of ring B outside its inner edge. They 

 are not the same as those measured by Barnard in 

 1907, which were on the outer part of ring B and on 

 the crepe ring. 



The position angle of the ring was measured on 

 February 22 ; the value found was 85° 14', which is 

 7' less than the Nautical Almanac value. Measures of 

 Saturn's disc gave for the equatorial diameter 17-65'', 

 and for the polar one 15-75"; compression, 1/9-3. ^^ 

 is interesting to note that the equatorial horizontal 

 parallaxes of the sun from the earth and Saturn are 

 practically identical. 



The ring will be edgewise to the sun on April 10 ; 

 after that its dark side will again be turned towards 

 the earth until August 3, when the third passage 

 through the ring plane will take place. 



Brazilian National Observatory Annual. — The 

 Annuario pelo Observatorio Nacional do Rio de 

 Janeiro for 192 1 contains the usual astronomical 

 data, together with expanded refraction tables and a 

 verv extensive list of useful constants. There are a 

 full description, with diagrams, of the various wire- 

 less time signals, and an essay on the calendar, 

 describing the various suggestions that have lately 

 been put forward for eliminating the inconveniences 

 of the present system. 



The magnetic elements for a large number of 

 Brazilian stations are given. Those for Rio de Janeiro 

 are tabulated at twenty-year intervals from 1660 on- 

 wards and compared with various formulae. The 

 latest formula for magnetic declination is that due 

 to Dr. Morize, the present National Astronomer, viz. 

 5-6°+o-o8°f+8o° sin (0-63° f- 44-1), t being reckoned in 

 vears from 1850. The largest residual of this formula 

 is 044° in 1760. The sine-term has a period of 

 571 years. 



Tide tables for nine Brazilian ports complete the 

 volume. 



