836 



SCIENCE 



[N. S. Vol. XXXVII. No. 961 



discharge from the fine wire. In this corona 

 discharge the air molecules are dissociated 

 into ions and these charged ions quickly at- 

 tach themselves to the particles of dust or tar. 

 The intense electric field between the two elec- 

 trodes then drags the particles of dust or tar 

 to the large electrode, where they are de- 

 posited. 



W. S. Franklin 



A LOCAL MAGNETIC STORM 



In Science, of March 21, reference is made 

 to a paper just published by the Academy of 

 Science of St. Louis with the above title. In 

 this paper evidence is presented to show that 

 atmospheric ions tend to set like magnets 

 along the lines of the earth's magnetic field. 

 The effect of gusts of wind in disturbing these 

 ions, and in thus producing continual sway- 

 ing of the lines of force due to variations in 

 permeability, is pointed out. 



A more local and somewhat similar mag- 

 netic storm may be artificially produced as fol- 

 lows: 



Suspend a needle on a silk fiber. Provide it 

 with a mirror, telescope and scale. Partially 

 compensate the effect of the earth's field by 

 bar magnets set in parallel position. Place 

 two bar magnets on opposite sides of the 

 needle, as in the Gaussian method of deflec- 

 tion. Place a plate of glass over one mag- 

 net, and sprinkle iron filings upon it. The 

 defiecting effect of that magnet is increased. 

 The needle no longer lies in the magnetic 

 meridian. Balance the effect on the needle 

 by adjustment of the other deflecting magnet 

 and tap the plate. The permeability of the 

 space around the magnet is again increased. 

 A new readjustment may be made. Disturb 

 the iron filings by means of a brush, applied 

 to any small area of the plate. A magnetic 

 storm is thus produced. If the filings were 

 free to move without friction, they would all 

 respond to the disturbance. The needle does 

 respond. If the filings are made to accumu- 

 late near the poles, the deflecting effect of the 

 magnet is greatly increased. If the magnet 

 is supported at its middle part so that it is 



lifted above the plate of glass, the poles may 

 be loaded with iron filings. The apparent 

 magnetic moment of the bar may thus be 

 increased about 8 or 10 per cent. Such a 

 magnetic storm as is thus produced in the sur- 

 rounding space appears to be similar to that 

 produced in the field of the earth, when at- 

 mospheric ions accumulate around the mag- 

 netic poles of the earth. If any of these 

 Faraday lines are disturbed, they are all dis- 

 turbed. The balanced needle tells the story. 



It seems very probable that the daily varia- 

 tions in the earth's field may be explained as 

 due to this change in permeability brought 

 about by ionization of the air by sunlight. The 

 lines of force sway in opposite directions dur- 

 ing the forenoon and afternoon of each day, 

 their lateral motion being greatest in the equa- 

 torial belt. There is also apparently a similar 

 swaying in a vertical direction. 



In the forenoon the north end of the needle 

 swings towards the west in the northern hemi- 

 sphere, while the south pole swings towards the 

 west in the southern. In the equatorial belt 

 the needle suffers no change. These daily 

 variations are modified by summer and winter 

 conditions, as they should be if the above ex- 

 planation is valid. 



Francis E. Nipher 



PLUS AND minus 



In a review of my book, " On the Founda- 

 tion and Technie of Arithmetic," in Science, 

 April 18, 1913, Professor Cajori, after quoting 

 a sentence, says : 



In view of the fact that historians have been in 

 doubt as to the exact origin of + and — , the 

 authority for Halsted 's categorical statement would 

 be interesting. 



Hoping the readers of Science may be of 

 the professor's mind, I venture an outline. 



Minus, as the oral rendering of the symbol 

 — , takes a sense which did not exist in Latin 

 of any period. Murray says it probably orig- 

 inated in the commercial language of the 

 middle ages. In Germany the Latin words 

 plus and minus were used by merchants to 

 mark an excess or deficiency in weight or 

 measure. The earliest known examples of the 



