1905.] A Pertinacious Current. 79 



WEEKLY EYENINa MEETING, 



Friday, March 24, 1905. 



Sir William Crookes, D.Sc. F.R.S., Honorary Secretary 

 and Vice-President, in the Chair. 



Sir Oliver Lodge, LL.D. D.Sc. F.R.S. M.R.I., Principal of the 



University of Birmingham. 



A Pertinacious Current; or, the Storage of High-tension 

 Electricity by means of Valves. 



It is well known to physicists and engineers that currents of electricity 

 can be of three principal varieties. The first and oldest variety is a 

 continuous or steady current, of constant strength in one direction, 

 like a river. By such a current a great quantity of electricity can be 

 conveyed from place to place, though under conditions that it is 

 easily stopped by any trivial obstacle, either an accidental bad joint 

 or a purposed switch or interrupter — which is a famihar arrangement 

 for introducing into the stream an air-gap or other narrow non-con- 

 ducting obstruction, and thereby completely stopping the flow, save 

 at the first moment of attempted stoppage, when the impetus or 

 momentum of the current succeeds for an instant in bursting through 

 the obstacle, with spark and flame. 



The second variety is an intermittent or jerky current ; which is 

 analogous to the supply of water by an ordinary intermittent pump,, 

 such as a fire-engine or a garden-engine without its air-chamber, from 

 whose nozzle the water issues in jerks, unless there is some elastic 

 reservoir or chamber of variable capacity in which it can be stored 

 under pressure, and out of which it can emerge with fair regularity. 



The third variety is the important case of the well-known " alter- 

 nating current " ; wherein there is no progression of electricity at all, 

 but simply a surging or oscillation to and fro, maintained by a rapidly 

 reversed force of propulsion, such as is seldom applied to Hquids ; 

 though it is applied to solids in many forms of reciprocating machine, 

 and in several other oscillating or vibratory examples, of which the 

 best-known variety is concerned with musical instruments. An alter- 

 nating current of liquid, however, occurs in Nature, on a large and 

 slow scale, in the tides ; and it may be set up on a small scale in 

 a churn. 



An alternating electric current is characteristically produced by 

 nearly all the magnetic methods of exciting a current discovered by 

 Faraday, i.e. by those methods which generate a current by means of 

 a combination of magnetism and motion, as exemplified in the ordinary 



