May 1 8, 1888.] 



SCIENTIFIC NE\VS. 



473 



of ^^aper^i, Hetture^, ttu 



ROYAL INSTITUTION. 

 At the Friday evening meeting on April 27th, Mr. T. 

 Wimshurst delivered a lecture on " Influence Machines." 



The lecturer said that in 1762 Wilcke described a 

 simple apparatus which produced electrical charges by 

 influence, or induction, and following this the great 

 Italian scientist, Alexander Volta, in 1775 gave the elec- 

 trophorus the form which it retains to the present day. 

 This apparatus may be viewed as containing the germ of 

 the principle of all influence machines yet constructed. 



Another step in the development was the invention of 

 the doubler by Bennet in 1786. He constructed metal 

 plates, which were thickly varnished, and were supported 

 by insulating handles, and which were manipulated so as 

 to increase a small initial charge. 



In 1787 Carvallo discovered the very important fact 

 that metal plates, when insulated, always acquire slight 

 charges of electricity ; following up those two important 

 discoveries of Bennet and Carvallo, Nicholson in 1788 

 constructed an apparatus, having two discs of metal insu- 

 lated and fixed in the same plane. Then, by means of 

 a spindle and handle, a third disc, also insulated, was 

 made to revolve near to the two fixed discs, metallic 

 touches being fixed in suitable positions. With this 

 apparatus he found that small residual charges might 

 readily be increased. It is in this simple apparatus that 

 we have the parent of influence machines. 



In 1823 Ronalds described a machine in which the 

 moving disc was attached to and worked by the pendulum 

 of a clock. It was a modification of Nicholson's doubler, 

 and he used it to supply electricity for telegraph working. 

 For some years after these machines were invented no 

 important advance appears to have been made, and this 

 may be attributed to the great discoveries in galvanic 

 electricity which were made about the commencement of 

 this century by Galvani and Volta, followed in 1831 to 

 1857 by the magnificent discoveries of Faraday in electro- 

 magnetism, electro-chemistry, and electro-optics, and no 

 real improvement was made in influence machines till 

 i860, in which year Varley patented a form of machine 

 designed for telegraph working. 



In 1865 the subject was taken up with vigour in 

 Germany by Toepler, Holtz, and other eminent men. 



In 1866 Bertsch invented a machine, but not of the 

 multiplying type; and in 1867 Sir William Thomson 

 invented a form of machine which, for the purpose of 

 maintaining a constant potential in a Leyden jar, is ex- 

 ceedingly useful.' 



The Carr6 machine was invented in 1868, and in 

 1880 the Voss machine was introduced, since which 

 time the latter has found a place in many laboratories. 

 It closely resembles the Varley machine in appearance, 

 and the Toepler machine in construction. 



The Holtz machine has one revolving plate, the 

 second plate being fixed. The fixed plate is so much 

 cut away that it is very liable to breakage. Paper 

 inductors are fixed upon the back of it, while opposite 

 the inductors, and in front of the revolving plate, are 

 combs. To work the machine (i) a specially dry at- 

 mosphere is required ; (2) an initial charge is necessary ; 

 (3) when at work the amount of electricity passing 

 through the terminals is great ; (4) the direction of the 

 current is apt to reverse ; (5) when the terminals are 



opened beyond the sparking distance the excitement 

 rapidly dies away; (6) it does not part with free 

 electricity from either of the terminals singly. 



It has no metal on the revolving plates, nor any metal 

 contacts ; the electricity is collected by combs which 

 take the place of brushes, and it is the break in connec- 

 tion of this circuit which supplies a current for ex- 

 ternal use. 



The first type of Holtz machine has now in many in- 

 stances been made up in multiple form, within suitably con- 

 structed glass cases, but when so made up great difficulty 

 has been found in keeping each of the many plates to a 

 like excitement. When differently excited the one set of 

 plates furnished positive electricity to the comb, while 

 the next set of plates gave negative electricity — as a con- 

 sequence, no electricity passed the terminals. 



To overcome this objection, to dispense with the dan- 

 gerously cut" plates, and also to better neutralise the re- 

 volving plate, throughout its whole diameter, I made a 

 large machine having twelve discs 2 ft. 7 in. in diameter, 

 and in it I inserted plain rectangular slips of glass 

 between the discs, which might readily be removed ; 

 these slips carried the paper inductors. To keep all the 

 paper inductors on one side of the machine to a like 

 excitement, I connected them together by a metal wire. 

 The machine so made worked spendidly, and your late 

 President, Mr. Spottiswoode, sent on two occasions to 

 take note of my successful modifications. The machine 

 is now ten years old, but still works splendidly. 



The Carre machine consists essentially of a disc of 

 glass which is free to revolve without touch or friction. 

 At one end of a diameter it moves near to the excited 

 plate of a frictional machine, while at the opposite end of 

 the diameter is a strip of insulating material, opposite 

 which, and also opposite the excited amalgam plate, are 

 combs for conducting the induced charges, and to which 

 the terminals are metallically connected ; the machine 

 works well in ordinary atmosphere, and certainly is in many 

 ways to be preferred to the simple frictional machine. 

 The quantity of electricity may be more than doubled by 

 adding a segment of glass between the amalgam cushions 

 and the revolving plate. The current in this type of 

 machine is constant. 



The Voss machine has one fixed plate and one revolv- 

 ing plate. Upon the fixed plate are two inductors, while 

 on the revolving plate are six circular carriers. Two 

 brushes receive the first portions of the induced charges 

 from the carriers, which portions are conveyed to the 

 inductors. The combs collect the remaining portion of 

 the induced charge for use as an outer circuit, while the 

 metal rod with its two brushes neutralises the plate sur- 

 face in a line of its diagonal diameter. When at work it 

 supplies a considerable amount of electricity. It is self- 

 exciting in ordinary dry atmosphere. It freely parts 

 with its electricity from either terminal, but when so 

 used the current frequently changes its direction, hence 

 there is no certainty that a full charge has been obtained, 

 nor whether the charge is of positive or negative 

 electricity. 



The construction of the Wimshurst machine 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 it two brushes 

 supported by metal rods, the rods to the two plates form- 

 ing an angle of 90 deg. with each other. The external 

 circuit is independent of the brushes, and is formed by 

 the combs and terminals. 



