62 



Electrostatics Field of Force 



[CH. ii 



place from the point of the lightning conductor sooner than from any part of 

 the building, and by putting the conductor in good electrical communication 

 with the earth, it is possible to ensure that no harm shall be done to the 

 main buildings by the electrical discharge. 



An application of the same principle will explain the danger to a human 

 being or animal of standing in the open air in the presence of a thunder 

 cloud, or of standing under an isolated tree. The upward point, whether the 

 head of man or animal, or the summit of the tree, tends to collect the lines of 

 force which pass from the cloud to the ground, so that a discharge of elec- 

 tricity will take place from the head or tree rather than from the ground. 



FIG. 26. 



72. The property of lines of force of clustering together in this way is 

 utilised also in the manufacture of electrical instruments. A cage of wire is 



Fm. 27. 



placed round the instrument and almost all the lines of b force from any 

 charges which there may be outside the instrument will cluster together on 

 the convex surfaces of the wire. Very few lines of force escape through this 

 cage, so that the instrument inside the cage is hardly affected at all by any 

 electric phenomena which may take place outside it. Fig. 27 shews the 

 way in which lines of force are absorbed by a wire grating. It is drawn to 

 represent the lines of force of a uniform field meeting a plane grating placed 

 at right angles to the field of force. 



