'80 Sir Oliver Lodge [March 24, 



•dynamo. It is true that these currents can be rectified, and so trans- 

 mitted in one direction over a portion of the circuit, by means of 

 some kind of commutator ; but such an arrangement never operates 

 over the tvhole of the circuit ; there is nearly always one part of the 

 •circuit, and that the generating part, where the quantity of electricity 

 oscillates equally to and fro. The so-called " unipolar " machines are 

 an exception. It is, however, possible to interpose something in the 

 path of an alternating current so as to prevent the passage one way 

 and permit it the other, that is to say, to introduce into the circuit a 

 one-sided kind of conductivity, such as is possessed by a trap or valve, 

 which permits ingress but prevents egress — a kind of gate, such as 

 is sometimes used for public gardens or parks, whereby people can go 

 out but not come in. Or like a mouse-trap, which lets creatures in 

 but will not let them out. 



If such an arrangement exists in an alternating-current circuit, it 

 •changes the current into an intermittent or jerky one, with the pro- 

 gress either wholly in one direction or more in one direction than in 

 the opposite. Such an arrangement may be conveniently called an 

 electrical valve or trap. By the use of such valves I have found it 

 possible to store up electricity, supplied by intermittent jerks, in a 

 reservoir, until the tension is raised to a high value ; and it can then 

 be allowed to leak or overflow in a constant continuous stream or 

 trickle, which, though not transmitting a very large quantity of elec- 

 tricity, can nevertheless overcome very considerable obstacles, pertina- 

 ciously flowing in spite of opposition, like a stream down a steep 

 hillside. This is what I call for the moment a pertinacious current. 

 It could always be produced by means of an electrostatic machine — 

 either the old-fashioned f rictional machine, or a Holtz or Wimshurst 

 inductive machine — and about the pertinacity of such a current there 

 was no dispute ; but unless such machine were of enormous size, the 

 ■quantity propelled was very small, and the current was essentially a 

 weak one. Moreover the generating machine was necessarily of a 

 delicate laboratory description, such as could hardly be regarded as 

 appropriate for engineering practice on a large scale. 



It has been always theoretically possible also to produce a high- 

 tension or pertinacious current by means of a voltaic battery of an 

 enormous number of cells ; and by some experimenters, such as 

 De la Rue and others, a battery of this kind was actually employed. 

 In the case of a voltaic battery the quantity put in motion is con- 

 siderable, but the difficulty was to raise the propelling force to the 

 required amount — usually it is very weak ; and in order to imitate 

 such effects as are easily producible by a large Wimshurst machine, 

 some considerable fraction of a million would be the number of cells 

 necessary. The expense and trouble of such a battery would be 

 prohibitory to most people, and to most undertakings ; especially since 

 .the cells have but a temporary and rather brief life. 



By the use of electric valves, however, I find it possible to employ 



