PHYSICS. 383 



arranged that in an ordinary atmosphere the colorless lamp alone shines, 

 but in fire damp this goes out and the red one lights up. This is effected 

 in a simple manner by the motion of a mercury contact opening the 

 lower part of a curved tube, one end of wliich is open and the other con- 

 nected with a porous pot of unglazed porcelain, the motion of the mer- 

 cury being due to the increased pressure in the pot caused by diffusion. 

 (Nature, July 188G, xxxtv,210; Phil. Mag., August 1886, V, xxii, 145.) 



Von Lang has measured directly the counter electro-motive force of 

 the voltaic arc as follows : Two sei)arate sets of light apparatus are ar- 

 ranged symmetrically in the circuit of a Benson battery so that the 

 middle of the battery and the middle of the conductor uniting the two 

 lamps are at the same potential. By the aid of a Wheatstone bridge 

 the resistance of the two portions of the circuit are determined ; first 

 when both lamps are in action, again when the carbons have been re- 

 placed by resistances which restore the current to its primitive value. 

 The half-difference of the two values thus obtained gives the resistance, 

 which is compensated by the counter electro-motive force of one of the 

 voltaic arcs. This is readily calculated by multiplying the difference 

 obtained by the current strength. The value thus found is 38.6 volts. 

 (Anz. Ak. Wien, 1885, 89 ; J. Phys., May, 1886, II, v. 239, November, 

 542.) 



Cross and Shepard have also studied the counter electro-motive force 

 of the arc, under various conditions. They conclude: (1) That there 

 is adefinite inverse electromotive force for the whistling arc, whose value 

 is approximately 15 volts ; (2) that the inverse electro-motive force for 

 both the silent and whistling arcs diminishes slowly as the current in- 

 creases; (3) that the inverse electro-motive force, at least for the whist- 

 ling arc, is less for the inverted than for the upright arc ; (4) that the 

 great change in equivalent resistance which occurs when volatile salts 

 are introduced into the arc is chiefl^'^ due to a large fall in the inverse 

 electro-motive force, although there is at the same time a marked dim- 

 inution in the conductive resistance; (5) that tlie diminished total re- 

 sistance of the arc in rarefied air is due solely to a diminution in the 

 conductive resistance; and (6) that there is some evidence to show 

 that with considerable reduction of pressure there is a slight increase 

 in the inverse electro-motive force. (Proc. Am. Acad., 1886, xxii, 227.) 



Oarhart has examined the question of electrical surface transmission 

 and concludes there is no sufficient scientific basis for making lightning 

 conductors of large surface, and that large sectional area is essential to 

 ample conductivity. (Am. J. Sci., April, 1886, III, xxxi, 256.) 



Douglass has suggested the use of fluted carbons 50""" in diameter 

 for light-house electric lighting. When in use no crater is formed in 

 either of the carbon points and their form is all that can be desired for 

 utilizing fully the maximum light of the radiant arc. He estimates that 

 the actual gain in light by their use is not less than 10 per cent. (Na- 

 ture, July, 1886, xxxiv, 209.) 



