PHYSICS. 617 



published. An abstract of it has been made by Potier. (Ann. Ghim. 

 Phys., May, 1883, V, xxix, 5; J. Phys., January, 1883, II, n, 11.) 



Spr;?gue has published the results of his tests made upon the new 

 form of Edison-Hopkinson dynamo, the peculiarity of which is its short- 

 ened field magnets. The resistance of the armature cold was 0.026, 

 and hot 0.0325 ohm. The mean of three experiments gave a total effi- 

 ciency of 94.8 per cent., and a commercial efficiency of 86 per cent. 

 (Nature, August, 1883, xxvin, 405.) 



4. Electric Spark and Electric Light. 



Villari has experimented upon the sparks from a condenser, and upon 

 the modifications which they undergo when various resistances are 

 inserted in the circuit. When a condenser is discharged so as to produce 

 at first a single spark, and then two in series, it is observed that the 

 former is not equal to the sum of the latter in length, nor is it constant 

 in value. The author finds that with his apparatus (which he calls a 

 spintherometer) if one of the sparks is zero the other has its miuimum 

 value, 26 mm . When it becomes 2 mm or less, the sum becomes 40 mm , 

 a maximum. Between 3 and 30 mm the sum is constant at about 32 mm . 

 Hence it appears that a very small spark produced in the circuit of a 

 condenser has the singular property of lengthening a second spark 

 produced simultaneously in the same circuit. The effect of introducing 

 various kinds of resistance in the path of the short spark is given. 

 (J. Phys., June, 1883, II, n, 272.) 



VYackter has discovered that the electric spark is produced always by 

 only one of the two electricities, and hence that the carrying of solid par- 

 ticles, which constitutes this spark, is effected sometimes by positive, 

 sometimes by negative, electricity, and therefore always in one definite 

 direction. He finds that positive electricity can give a spark ouly when 

 the pressure of the air is above 10 mm of mercury. Negative electricity 

 can give a spark under pressures included between 63 and 5 mm , accord- 

 ing to the distance of the electrode from the wall of the tube. As the 

 air becomes more rarefied, the matter transported decreases from the 

 positive and increases from the negative electrode. Positive electricity 

 transports the particles much farther than negative. Under a pressure 

 of 63 mm , the positive spark can cross a space of 2,040 mm ; the negative 

 one only of 0.6 mm , or 3,400 times less. The positive particles follow the 

 line of least resistance, and hence may describe a curvilinear trajectory ; 

 the negative particles are thrown off normally, and move in straight 

 lines. A powerful magnet acts on the former as on diamagnetic bodies; 

 on the latter as on paramagnetic substances. The positive particles 

 sometimes become incandescent, and are measurable under the micro- 

 scope; the negative particles are never iucandescent, and are too tenu- 

 ous to measure. ( Wied. Ann., xvn, 903; J. Phys., June, 1883, II, n, 283.) 



Edlund has given the results of his experiments in favor of the 

 hypothesis advanced by him, that a vacuum opposes a very feeble re- 



