292 Dr. E. H. Cook on an Undulatory Movement 



which they have taken up, it is easily done by coating the 

 plate beforehand with a thin layer of gum, and after the 

 experiment breathing on it. 



3. Effect of Plate. — When the figures were first obtained 

 it was surmised that they might be caused by the distribution 

 of the electricity over the plate ; that they were in fact modi- 

 fied Lichtenberg's figures. Experiments were therefore made 

 with plates of various substances in different states as regards 

 their surfaces; for example, whether polished or dull, elec- 

 trified or unelectrified. The following materials have been 

 examined : — glass ; resinous cake of electrophorus (a mixture 

 of shell-lac, resin, and Venice turpentine) ; ebonite ; paper 

 (glazed and ungiazed) ; brass (lacquered and unlacquered) ; 

 zinc ; iron, tinned ditto, galvanized ditto; wood; paraffin; card- 

 board ; and glazed and ungiazed earthenware. Except that the 

 rougher surfaces retard and prevent the regularity of the 

 curves, no difference whatever could be observed as due to 

 the substance or condition of the surface. 



A piece of plate-glass or polished sheet of metal answers 

 admirably for these experiments. The fact that the produc- 

 tion of the curves is independent of the electrical state of the 

 support (and therefore of the powder) justifies the statement 

 that they are not caused by any peculiarity of electrical 

 distribution. 



4. Effect of Battery Power, Coil, and Condensation. — 

 Experiments were made with the view of finding if variations 

 in the number of cells, in the coil, and in the capacity of the 

 condensers used, caused any alteration in the shape or 

 frequency of the lines in which the powders arranged them- 

 selves. Other conditions remaining constant, the battery 

 power employed to drive the coil was changed from ten pint 

 bichromates to one. In no case could any difference be 

 observed in the resulting figures produced. Afterwards the 

 current was still further reduced until it was just sufficient 

 to drive the coil, but so long as this was accomplished the 

 figures remained unaffected. 



Several coils were used in the course of the earlier experi- 

 ments, but finally the work was confined to two. One of 

 these was a very fine instrument by Ruhmkorff, kindly lent to 

 me for the purpose of these experiments by Mr. Francis J. 

 Fry, late High Sheriff of Bristol. It contains a secondary 

 coil, formed of twenty-four miles of wire, is fitted with both 

 spring and mercury interrupters, and with one bichromate 

 (carbons 7" x 3J"), gives a regular succession of sparks six 

 inches long. The second was a smaller instrument, by Paterson 

 of London, fitted with a condenser of 100 sheets of 12" by 5", 



