230 Sir David Salomons, Bart. [J\me 21, 



hausted air, and the current employed is an alternate current. The 

 experiments here mentioned must, therefore, be regarded as a first 

 instalment. They will have to be repeated, with an intermittent 

 direct current and with tubes containing various gases, and probably 

 also tubes containing various vapours, and all these at various ex- 

 haustions and temperatures. 



I have already made a large number of experiments employing 

 tubes containing different gases and with direct, as well as alternate 

 current. I may, therefore, mention that the phenomena about to be 

 described, when using the alternate current, appear to be the same 

 with the direct current, except that the phenomena peculiar to direct 

 currents, in the form of the bands, make themselves apparent. But, 

 until these experiments have been completed, I will not refer to them 

 any further in this paper. I find it necessary to work in the con- 

 trary manner to that which is usually adopted. 



1. The alternations are made so slow that blinks are produced in 

 the tube under observation, which correspond to the reversals ; and 

 then the alternations are increased in speed until the tubes appeal- 

 continuously lightened. This is my starting point. I have the 

 means of raising the frequency when desired. The apparatus for 

 producing the alternate current, in the first instance, is a Pyke and 

 Harris alternator driven by an electromotor, hence as the frequency 

 increases so does the electromotive force ; I am therefore obliged, 

 when describing the experiments, to employ the expression " electric 

 energy " or " current " in order to avoid confusion. 



2. Since I use currents of such low frequency the electromotive 

 force also is very low ; and the quantity of current traversing any 

 tube is small. 



3. The tubes are not very highly exhausted, they are only ex- 

 hausted to approximately 0'5 mm. of mercury, according to the usual 

 mode of comparative measurement. But 1 find that, whether the 

 tubes are highly exhausted or not, provided the current passes, all 

 the phenomena are the same. At a very early stage of the investi- 

 gation I observed that the ordinary methods of working, t.e., with 

 currents of high electromotive force, masked the effects I was 

 seeking. 



In giving the subject a logical sequence so that each step may be 

 noted, I have no doubt that I shall describe several effects which are 

 already known. But to omit these, which I am unable to single out, 

 would be to lose the thread of the history. 



In a scientific paper it is usual to prove some definite point or to 

 show some special new phenomenon. It is therefore desirable to 

 point out the object aimed at in this communication. 



The object I originally had in view was simply to discover a. 

 method by which vacuum-tubes could be made to give a prede- 



