TRANSACTIONS OF SECTION A. 633 



tive electrification when pieces of copper foil, and negative electrification when 

 pieces of carbon, were placed in the tube, and when the temperature was sufficient 

 to powerfully oxidise the copper or to burn away the charcoal. 



§ 12. Through the kindness of Mr. E. Matthey, we have been able to experi- 

 ment with a platinum tube 1 metre lone and 1 mm. bore. It was heated either 

 by a gas flame or an electric current. When the tube was cold, and non-electri- 

 iied air drawn through it, we found no signs of electrification by our receiver and 

 electrometer. But when the tube was made red or white hot, either by gas 

 burners applied externally or by an electric current through the metal of the tube, 

 the previously non-electrified air drawn through it was found to be electrified 

 strongly positive. To get complete command of the temperature we passed a 

 measured electric current through -0 cm. of the platinum tube. On increasiug the 

 current x,ill the tube began to be at a scarcely visible dull red heat we found but 

 little electrification of the air. "When the tube was a little warmer, so as to be 

 quite visibly red hot, large electrification became manifest. Thus 60 strokes of the 

 air-pump gave 45 scale divisions on the electrometer when the tube was dull red, 

 and .395 scale divisions (7 volts) when it was a bright red (produced by a current 

 of .36 amperes). With stronger currents, raising the tube to white-hot tempera- 

 ture, the electrification seemed to be considerably less. 



2. Bo Vertical (Earth-Air) Electric Currents Exist in the United 

 Kingdom ? By A. W. Rucker, F.R.S. 



In a paper by Dr. Adolph Schmidt read before Section A of the British 

 Association at Oxford,' the author stated that he had expanded the components of 

 the earth's magnetic force in series, and had deduced expressions, two of which 

 give the magnetic potential on the surface of the earth in so far as it depends on 

 (1) internal and (2) external forces. ' The third series represents that part of the 

 magnetic forces which cannot be expressed in terms of a potential, but must be 

 due to electric currents traversing the earth's surface.' The author concludes that 

 such currents amount ' on the average to about 0"! ampere per square kilometre.' 



It appeared, therefore, desirable that this conclusion, drawn from the magnetic 

 state of the earth as a whole, should be tested by means of those portions which 

 have been most fully studied. 



The test to be applied is whether the line integral of the magnetic force taken 

 round a re-entrant circuit is or is not a vanishing quantity. 



The irregular form of the United Kingdom makes tlie application of this test 

 more difficult than it would otherwise be, but as two detailed surveys of Great 

 Britain and Ireland have been carried out by Dr. Thorpe and myself for the 

 epochs 1886 and 1891 respectively, the data at our disposal are so numerous that 

 I thought it worth while to undertake the inqtury. The actual work of calcida- 

 tion has been carried out almost entirely by two of my students, Messrs. Kay and 

 AVhalley. My best thanks are due to them for the care and skill they have 

 displayed. 



Two circuits called the a and /3 circuits respectively were selected, bounded by 

 the following lines : — 



(a) Long. 2° W., Lat. 58° N. ; Long. 7° W., Lat. 52° N. 

 (/3) Long. 1° W., Lat. 55° N. ; Long. 9° W., Lat. 52° N. 



The work done by a unit magnetic pole on traversing these circuits was calculated 

 for the epoch 18860 by means of the terrestrial lines found for that date, and also 

 for the epoch 1891-0 by means (1) of the same lines when due allowance was 

 made for the secular change, and (2) of the independent set of lines found by aid 

 of the later survey. 



The magnitudes of the hypothetical currents deduced from these calculations 



• Ren. Brit. Assoc, 1894, p. 570. 



