428 Mr. W. II. Preece on the Space 



sarilv perpendicular to this surface. When the lines of force 

 and the equipotential surfaces arc straight, parallel, and equi- 

 distant, we have a uniform field. The intensity of the field is 

 shown by the number of lines passing through unit area, and 

 the rate oi' variation of potential by the number of equipoten- 

 tial surfaces cutting unit length of each line of force. Hence 

 the distances separating the equipotential surfaces are a mea- 

 sure of the electromotive force present. Thus an electric field 

 can be mapped or plotted out so that its properties can be in- 

 dicated graphically. 



The air in an electric field is in a state of tension or strain ; 

 and this strain increases along the lines of force with the elec- 

 tromotive force producing it until a limit is reached, when a 

 rent or split occurs in the air along the line of least resistance 

 — which is disruptive discharge, or lightning. 



Since the resistance which the air or any other dielectric 

 opposes to this breaking strain is thus limited, there must be 

 a certain rate of fall of potential per unit length which corre- 

 sponds to this resistance. It follows, therefore, that the num- 

 ber of equipotential surfaces per unit length can represent this 

 limit, or rather the stress which leads to disruptive discharge. 

 Hence we can represent this limit by a length. We can pro- 

 duce disruptive discharge either by approaching the electrified 

 surfaces producing the electric field near to each other, or by 

 increasing the quantity of electricity present upon them ; for 

 in each case we should increase the electromotive force and 

 close up, as it were, the equipotential surfaces beyond the limit 

 of resistance. Of course this limit of resistance varies with 

 every dielectric; but we are now dealing only with air at ordi- 

 nary pressures. It appears from the experiments of Drs. 

 Warren De La Rue and Hugo MulJer that the electromotive 

 force determining disruptive discharge in air is about 40,000 

 volts per centimetre, except for very thin layers of air. 



If we take into consideration a flat portion of the earth's 

 surface, AB (Plate X. fig. 1), and assume a highly charged 

 thunder-cloud, C D, floating at some finite distance above it, 

 they would, together with the air, form an electrified system. 

 There would be an electric field ; and if we take a small 

 portion of this system, it would be uniform. The lines a b, 

 a!V . . . would be lines of force; and c dj c' d' ', c" d" . . . would 

 be equipotential planes. 



If the cloud gradually approached the earth's surface (fig. 2), 

 the field would become more intense, the equipotential surfaces 

 would gradually close up, the tension of the air would increase 

 until at last the limit of resistance of the air ef would be 

 reached; disruptive discharge would take place, with its atten- 



