804 JJr. J. Wimshursi [April 27, 



made, the first of which I presented to this Institntion. Its eon- 

 straction is of the simplest character. The two discs of glass revolve 

 near to each other, and in opposite directions. Each disc carries 

 metallic sectors ; each disc has its two brushes supported by metal 

 rods, the rods to the two plates forming an angle of 90" with each 

 other. The external circnit is independent of the brushes, and is 

 formed by the combs and terminals. 



The machine is self-exciting under all conditions of atmosphere, 

 owing probably to each plate being influenced by, and influencing in 

 turn it-s neighbour, hence there is the minimum surface for leakage. 

 When excited the direction of the current never changes ; this cir- 

 cumstance is due probably to the circuit of the metallic sectors and 

 the make-and-break contacts always being closed, while the combs and 

 the external circuit are supplemental, and for external use only. The 

 quantity of electricity is very large and the potential high. When 

 suitably arranged the length of spark produced is equal to nearly 

 the radius of the disc. I have made them from 2 inches to 7 feet in 

 diameter, with equally satisfactory results. The Diagram Xo. 9 

 shows the distribution of the electricity npon the plate surfaces, 

 when the machine is fully excited. The inner circle of signs corre- 

 sponds with the elc<-tricity upon the front surface of the disc. The 

 two circles of signs between the two black rings refer to the electri- 

 city between the discs, while the outer circle of signs corresponds 

 with the electricity upon the outer surface of the back disc. The 

 diagram is the result of experiments which I cannot very well repeat 

 here this evening, but in support of the distribution shown on the 

 diagram I will show you two discs at work made of a flexible 

 material, which when driven in one direction, close together at the 

 top and bottom, while in the horizontal diameter they are repelled. 

 When driven in the reverse direction the opposite action takes place. 



I have also experimented with the cylindrical form of the 

 machine ; the first of these I made in 1882, and it is before you. 

 The cylinder gives inferior results to the simple discs, and is more 

 complicated to adjust. You notice I neither use nor recommend vul- 

 canite, and it is perhaps well to caution my hearers against the use 

 of that material for the purpose, for it warps with age, and when left 

 in the daylight it changes and bc<:omes useless. 



I have now only to speak of these larger machines. They are in 

 all respc-cts made up with the same plates, sectors, and brushes as 

 were used by me in the first experimental machines, but for conve- 

 nience sake they are fitted in numbers within a glass case. 



This machine has eight jdates of 2 feet 4 inches diameter ; it has 

 been in the possession of the Institution for about three years. 



This large machine, which has been made for this lecture, has 

 twelve discs, each 2 feet 6 inches in diameter. The length of spark 

 from it is 13|^ inches. 



During the construction of the machine every care was taken to 

 avoid electrical excitement in any of its parts, and after its comjdetion 



