48 
error of observation. The work here referred to may not appear to ‘be 
very directly associated with the determination of standards of measure- 
ment. It is, however, one of the investigations which has been made 
possible by the work of the British Association committee in the produc- 
tion of instruments of precision. Prominent’ among these instruments 
stands the Kelvin electrometers and particularly the absolute electrometer 
which was described in the report of the British Association committee in 
1867. 
Another subject of great interest in itself and in connection with Max- 
well’s theory is that of the specific inductive capacity of dielectrics. 
Pxperiments on this subject were made by Faraday, but comparatively 
little was done before 1870, in which year an excellent paper was commu- 
nicated to the Royal Society by Gibson and Barclay on the specific induct- 
ive capacity of paraffin. Since that time much good work has been done by 
Boltzmann, Hopkinson, Quincke, Silow, Klemencie, Negreano and others. 
The theoretical importance of these experiments is due to the fact that ac- 
cording to Maxwell's theory the specific inductive capacity of nonmagnetic 
dielectrics should be proportional to the squares of their indices of refrac- 
tion. A wonderful verification of Maxwell’s theory was carried out only 
some ten years ago by Hertz, who showed not only that electrical waves 
exist, but also how to measure their wave length'and period. We have in 
these experiments splendid illustrations of the oscillatory discharge re- 
ferred to above as discovered by Henry and predicted by Thomson, and 
as a result several new ways of determining electrical quantities have 
been developed. It is now possible, for example, to compare the capacity 
of condensers by means of oscillatory currents of exceedingly short period 
and thus to determine the dielectric constants of many materials to which 
the older methods were not easily applicable. 
It is somewhat difficult to decide where to place a reference to the re- 
cent discovery of Réntgen and its developments in photography, but prob- 
ably it comes in well here. Just how to apply Maxwell's equations to 
Réntgen’s rays is not yet quite clear, but there is no doubt as to the great 
importance of the discovery. 
As an outcome of all this activity in the determination of standards 
and in the absolute measurement of the electric properties of materials, 
combined with the great commercial demand produced by the introduction 
of dynamo machinery, we have now many excellent instruments at our 
