142 Scientific Intelligence. 
present. Thus an electric field can be mapped or po out so 
that its ens can be indicated graphically. 
n an electric field is in a state of tension or strain; an nd 
the stress which leads to pg discharge. Hence we can 
represent this limit by a length. We can produce disruptive dis- 
charge either by approaching the electrified surfaces producing 
the electric field near Ags each other, or by i poicgeet the ana 
of electricity present upon them ; for in —° we shoul 
increase the Seip aneoTA force and close up, as <u were, the 
pr ong Ng surfaces beyond the limit of emetained: Of course 
this limit of resistance varies with every dielectric; but we are 
now desling only with air at ordinary pressures. It appears from 
the experiments of Drs. Warren De La Rue and Hugo Miller 
that the electromotive force determining pe ee discharge in 
air is pt 40,000 volts per centimeter, except for very thin 
layers of ai 
If we ree into consideration a flat portion of the earth’s sur 
face, A B (fig. 1), and assume a highly charged thunder-cloud, 
CD, floating at some finite detenea shove it, they would, together 
Fig I, 
== 
‘ D 
ees 4 
2 aa g d 
Rae PR i 
c _— a 
rol a’ 
A > 3 B 
Leer the air, fons an electrified system. There would be an n elec- 
c field; and if we take a small portion of this system, it woul 
be ‘uniform. The —— ba'b’. would = — of force; a0 nd 
cd,c' d',c’ ad’ . uld be + equipolniial plan 
If the cloud gradually approached ~ carts surface (fig. 2 
the field would become more intense e equipotential surfaces 
would evidond ly close up, the tension of the air would increase 
until at last the limit of resistance of the air e f would be reached ; 
disruptive discharge would take place, with its attendant thunder 
