PHYSICS. 483 



Ducretet'a instrument has the two astatic ueedles iu the same hor- 

 izontal plane. (C. R., October, 1884, xcix, G05.) 



Stone has described an electro-dynamometer with au extremely light 

 suspended coil made of aluminum wire. {Natiire, xxx, 035, October, 

 1884.) 



Bellati has substituted for the needle of a reflection galvanometer a 

 small cylinder of soft iron suspended bifilarly in a direction i)erpeudicu- 

 lar to the magnetic meridian, the vertical coil through which the current 

 passes being inclined 45° to this meridian. Since the electro-magnetic 

 action preserves always the same sign, the instrument acts as an elec- 

 tro-dynamometer. If a bundle of soft iron wires be used, 0.15°'" in diam- 

 eter and IT'""" long, the instrument readily indicates telephone cur- 

 rents. (J. Phys., May, 1884, If, iii, 220.) 



4. Electric SparJc and Electric Light. 



Foster and Pry son have, given a simple formula to represent the dif- 

 ference of potential required to give sparks in air. If this difference of 

 potential be represented by V, and the length of the spark in centimeters 

 be ?, their experiments give approximately V= 102^ + 7.07. The results 

 were obtained with brass balls 1.35"™ in diameter, africtional machine, 

 and a Foster absolute electrometer. When /=0.142, the difference of 

 potential was 154.7(); with ^=0.284, V= 133.35; for / = 0.497, V = 131.66; 

 and for I = 0.9, V = 138.57 ; the value of Y diminishing then and increas- 

 ing again. {Nature, March, 1884, xxix, 446.) 



In his Bakerian lecture, Schuster has ably discussed the experiments 

 which hav^e made on the electric discharge through gases, and has given 

 a sketch of a theory of this action. He shows that the negative glow 

 is divisible into three layers, increasing in thickness with decreasing 

 density. In the first place, a luminous layer closely surrounds the elec- 

 trode, which, when this is new, is of a golden color^ The spectroscope 

 shows in this the presence of sodium and hydrogen. The second layer is 

 known by the name of the dark space. Tbe third layer is the glow 

 proper. The theorj' proposed is this : that within the first layer the 

 gaseous molecules are decomposed, that their respective parts are pro- 

 jected with great velocity through the dark space, that this velocity is 

 gradually reduced by impacts within the glow, and that in the positive 

 part of the discharge the discharge takes place by diffusion, except when 

 stratifications appear. {Nature, July, 1884, xxx, 230.) 



E. Wiedemann has also investigated the phenomena of the electric 

 discharge iu gases. He supposes that the positive discharge consisting 

 in a current preceded by a polarization of the gas is an electrical phe- 

 nomenon proper, and he attributes it to a longitudinal motion. The 

 negative rays are the true luminous rays, of very short wave-length, 

 energetically absorbed by the ponderable medium, in which they excite 

 ])hosi)horescence. The negative electrode is a source of transversal 

 waves, exerting on their front, according to Maxwell, a pressure which 



