On the Space protected by a Lightning -Conductor. 427 



up. It will in this be necessary first of all to further examine 

 the proposition previously stated, and probably to expand it into 

 the following general statement : — 



" In order that definite changes in quality and quantity may 

 take place in the spectrum of a gas produced by electric dis- 

 charges, equal quantities of energy must be communicated to 

 each molecule, which are, within tolerably wide limits, inde- 

 pendent of the pressure of the gas and of the width of the 

 tube." 



Leipsic, March. 1880. 



XL VIII. On the Space protected by a Lightning- Conductor. 

 By William Henry Preece*. 



[Plate X,] 



ANY portion of non-conducting space disturbed by electri- 

 city is called an electric field. At every point of this 

 field, if a small electrified body were placed there, there would be 

 a certain resultant force experienced by it dependent upon the 

 distribution of electricity producing the field. When we know 

 the strength and direction of this resultant force, we know all 

 the properties of the field, and we can express them numeri- 

 cally or delineate them graphically. Faraday (Exp. Res. 

 § 3122 et seq.) showed how the distribution of the forces in 

 any electric field can be graphically depicted by drawing lines 

 (which he called lines of force) whose direction at every point 

 coincides with the direction of the resultant force at that point; 

 and Clerk Maxwell (Camb. Phil. Trans. 1857) showed how 

 the magnitude of the forces can be indicated by the way in 

 which the lines of force are drawn. The magnitude of the re- 

 sultant force at any point of the field is a function of the poten- 

 tial at that point; and this potential is measured by the work 

 done in producing the field. The potential at any point is, in 

 fact, measured by the work done in moving a unit of elec- 

 tricity from the point to an infinite distance. Indeed the re- 

 sultant force at any point is directly proportional to the rate 

 of fall of potential per unit length along the line of force pass- 

 ing through that point. If there be no fall of potential 

 there can be no resultant force ; hence if we take any sur- 

 face in the field such that the potential is the same at every 

 point of the surface, we have what is called an equipotential 

 surface. The difference of potential between any two points 

 is called an electromotive force. The lines of force are neces- 



* Communicated by the Author. 



