1 84 



NATURE 



[Dec. 26, 1878 



" It is obvious," he continues, "that the description given of 

 the phenomenon is incomplete, for ductility, elasticity, variation 

 of the resonance-plate, &c., co-operate to produce a more com- 

 plicated phenomenon. I have tried a great many materials for 

 undertones, and found that they fall, in this respect, into three 

 groups. In the middle stand those materials which furnish 

 undertones, that is, the great majority of all substances in general. 

 On the one side are those substances which, as soon as the vibra- 

 tions are pretty strong, give no resonance-tones, but merely an 

 indeterminate noise ; to this group belong rolled plate metal and 

 most kinds of glass. On the other side are those substances 



which, however strong the vibrations, always give the tone of 

 the tuning-fork. I have found only one example of this, viz., 

 the wood of mountain fir, in thin polished plates. It was 

 natural to try the belly of a violin, which is mostly made of fir- 

 wood, for undertones, and in this way form an idea as to the 

 elasticity of the wood, on which the excellence of the instrument 

 greatly depends. From the German violins I have examined, I 

 have always obtained undertones ; from the few authentic Italian 

 violins accessible to me I obtained, on the other hand, always 

 the original tone. But I acknowledge that more abundant 

 material is necessary for a decision of this question." 



THE DISRUPTIVE DISCHARGE OF ELEC- 

 TRICITY^ 



"D Y means of the following method we have been able to in- 

 "^ vestigate the laws of the disruptive discharge of electricity 

 of high potential — a subject of investigation which is the com- 

 plement of that in which Drs. Warren de la Rue and Miiller 

 have been simultaneously engaged. In making these experiments 

 I have had the able co-operation in succession of Messrs. 

 Salvesen, Connor, Stewart, Simpson, and Playfair. 



The method essentially consists in connecting the prime con- 

 ductor of the Holtz machine, not with the electrometer directly, 

 but with an insulated spherical ball placed at some distance from 

 .an equal spherical ball, the latter being connected with the 

 electrometer. The woodcut represents, in situ, the apparatus 

 which was used in the case of the gases. The receiver of the 

 air-pump, which has a rod capable of moving air-tight, was 

 attached to one of the conductors of the Holtz machine in such 

 a manner that the conductor and the rod formed one conducting 



system. Projecting from the plate of the air-pump was a short 

 metal rod, which formed one conductor with the metallic parts 

 of the air-pump, and, by means of a wire, with the uninsulated 

 conductor of the Holtz machine. Electrodes of various forms 

 were made to screw on to the ends of the rods. Of the two in- 

 sulated brass balls one. A, was fixed ; the other, B, could be 

 moved along the connecting board. The wire joining A to the 

 collar of the receiver is insulated with gutta-percha. The elec- 

 trometer in connection with b is one of Sir VV. Thomson's 

 divided ring reflecting electrometers. 



When the potential of A is raised by driving the machine, the 

 potential of B is also raised, and this goes on until a discharge 

 takes place between the electrodes inside the receiver. Hence 

 the maximum deflection of the spot of light from zero is an in- 

 dication of the difference of potential of the two surfaces between 

 which the spark passed immediately before the discharge. By 

 breaking the contact between the conductors of the Holtz 

 machine before beginning to turn the wheel, and, by turning 

 slowly and uniformly, we were able to make the image of the 



wire move up continuously, and to be at rest at the instant of 

 discharge. After the discharge took place the image fell back 

 to zero, or a point near zero. We always noted the position 

 taken up by the image when the conductor of the machine was 

 completely discharged. 



The force resisting the deflection of the mirror is the action of 

 two external magnets upon several small magnets fixed to the 

 back of the mirror. ^ 



One great merit of our method is the rapidity with which 

 observations can be made. Three readings were in general 

 taken for each entry. The mean of these is very probably free 

 from any error due to accidental variations in the passage of the 

 •spark. An extensive series of observations have been printed 



' Abstract by the author of thesis for D.Sc. and other papers printed in 

 the recently issued ' part of the Transactions of the Royal Society of 

 Edinburgh. By Alexander Macfarlane, M.A., D.Sc. 



* Our results were reduced to absolute measure by means of the abso- 

 lute electrometer represented on the table. 



in full in the Transactions of the Royal Society of Edinburgh. 

 The following are the more important results : — 



A spark was taken through air between plates at a constant 

 distance, and the distance between the balls A and b varied. 

 Let V denote the induced potential, and r the distance between 

 the centres of A and B ; then the experimental curve obtained 

 satisfies the equation — 



V — 6o8ir "^ -42'26 

 for values of r greater than 24 centimetres j"" but for less values 

 of r the function requires to he. corrected by being multiplied 

 by- 



•524 -f '02r. 



Sparks were taken through air at the atmospheric pressure 

 between parallel metal disks of 4 inches diameter for distances 

 up to I "2 centimetres. The function for V, the difference of 

 potential in terms of s, the length of the spark is — 



V = 66*94 s/{ J^ + '20Ss], 



