156 BELL SYSTEM TECHNICAL JOURNAL 



however the same one; another interval, another train of ions, and so 

 forth. The method of measurement which thus far I have been pre- 

 supposing conceals these individual events, giving merely the sum of all 

 the ions which come into being over a period of hours (or, at least, of 

 many minutes). Towards the end of the article I shall speak of other, 

 and very striking, methods which reveal them. 



Passing now to the experiments, I will take up first the various 

 groups of measurements made, each with a single instrument of the 

 ionization-chamber or electroscope type, at various heights above the 

 ground or various depths below the surfaces of lakes. 



Imagine, to begin with, a metal-walled air-filled box, having inside it 

 a pair of strips of gold-foil hanging side by side from a metal knob at 

 the end of an insulating rod, and in one wall a window through which 

 this "gold-leaf electroscope" can be seen. The metal wall is earthed, 

 and the leaves of gold are charged, usually by a metal rod so mounted 

 in a ground-joint in the wall that by turning the joint the rod can be 

 touched to the knob which holds the leaves. The wall is discon- 

 nected from earth, the box is set up in whatever place is chosen for the 

 measurement. The divergence of the gold-leaves is measured at the 

 beginning and at the end of a chosen interval of time. By calibrating 

 the device (I will not enter into the details of this process) one may 

 determine how much charge of one sign has been lost from the leaves, 

 how much charge of the opposite sign has been gained by the leaves, 

 in the time allotted. This is the ultimate datum. The process is then 

 repeated in other places. 



All of the experiments which I shall now describe are in principle 

 like this imagined one, but greatly improved in technical detail. 

 Gold-leaves would be too frail for an apparatus meant to be moved 

 about; for them are substituted sometimes a pair of quartz fibres 

 brought together at both ends, not at one only (Fig. 1) or else a 

 "string electrometer." Sometimes the observations are made fifteen 

 kilometres up in the air, or two hundred metres down under water. 

 It is not always convenient, sometimes not feasible, for the observer to 

 go with his instrument; it is not always safe to make the initial reading 

 before the apparatus is sent on its way, the final reading after it is 

 brought back. The observations must then be delegated to a mecha- 

 nism; the divergence of the fibres or the position of the string is 

 recorded by photography, perhaps at intervals of time previously 

 chosen and regulated by machine, perhaps continuously. The pieces 

 of apparatus designed to do these things, and meanwhile to survive 

 immersion or transportation to great heights and low temperatures, 

 are often prodigies of compactness. 



