378 PHYSICS. 



the otlier end is positive inside and negative without. The charges are 

 very high and the glass is frequently perforated. If discharge is effected 

 by alternate contact of the ends beautiful luminous effects are seen in the 

 tubes. {Nature, January, 1881, xxiii, 218.) 



Preece hao discussed the relation which exists between the length of 

 a lightning conductor and the space which it protects. He assumes the 

 data of De la Eue and Miiller, that to produce a spark one centimeter 

 long in air requires a difference of potential of 40,000 volts, and concludes 

 that a lightning-rod protects a conical space whose height is the length 

 of the rod whose base is a circle having its radius equal to the height 

 of the rod, and whose side is the side of a circle whose radius is equal to 

 the height of the rod. {PMl. Mag., Dec. 1880, V, x, 427 ; Am. J. ScL, 

 February, 1881, III, xxi, 141.) 



Leconte has observed that these conditions laid down by Preece are 

 to be regarded as minimum conditions, because the use of iiointed con- 

 ductors would certainly increase very considerably the area protected. 

 {Nature, February, 1881, xxiii, 38G.) 



Jamin has examined the counter-electromotive force developed in the 

 arc. This electromotive force is equal to 20 or 25 volts ; so that the prin- 

 cipal work of maintaining the arc appears to be spent in overcoming this 

 opposing force, and is not occasioned by the resistance of the arc itself, 

 which is small. This forms the difificulty of maintaining many arc-lights 

 in the same circuit with batteries, continuous current machines or ac- 

 cumulators; but with alternate current machines with a certain speed, 

 this counter-electromotive force reaches a minimum. It appears to be 

 due to the difference of temperature between the carbons, and as this 

 difference disappears when alternate currents are used, the inverse elec- 

 tro-motive force is diminished. (0. E., May, 1881, xcii, 1021; Am. J. 

 Sci:, July, 1881, III, xxii, 74.) 



Nipher has shown that the statement by Preece, that the quantity of 

 heat evolved in each of the electric lamps contained in the same circuit 

 varies as the inverse ratio of the square of the number of lamps, is true 

 only in the special case which he considers. If the lamps be arranged 

 in parallel circuits, each circuit containing a certain number of lamps, 

 the total quantity of heat produced in the lamps is independent of the 

 number of lamps, the quantity of heat in each lamp varying inversely 

 as their number. {J. Phys., February, 1881, x, 94.) 



Avenarius has patented a method of subdividing the electric light, 

 founded on the insertion of a polarizer in a secondary circuit connected 

 with each electric lamp. This polarizer consists of several voltameters 

 connected together. The current from the machine divides at the lamp ; 

 one part goes through it, while the other goes through the polarizer. 

 The intensity of the light in the lamps ujay be varied by inserting re- 

 sistance in the i)olarizers, and by increasing the number. The Individ- 



