100 STATIC ELECTRICITY 



on the table and put the cover on it, the cover and table thus 

 forming the plates of a condenser. Then connect the table with 

 the negative terminal, and the cover with the positive terminal, of a 

 voltaic battery of several cells, when charges will gather pro- 

 portional to the potential difference of the terminals. Disconnect 

 the wires, first from the table, and then from the cover, and lift 

 the cover. The charge on the table is then shared with the leaves, 

 which diverge by an amount to be noted. Now remove the 

 ebonite plate and put three very small pieces of ebonite of the 

 same thickness as the ebonite plate on to the table merely to serve 

 as spacing pieces, and place the cover on them. We now have 

 an air condenser with the plates the same distance apart as 

 before, for the spacing pieces occupy a very small fraction of the 

 volume and may be neglected in a rough experiment. On con- 

 necting and then disconnecting the terminals of the battery as 

 before and lifting the cover we note that the leaves diverge much 

 less than before, or the table has received a much smaller charge 

 when air is the dielectric than it received when ebonite was the 

 dielectric. The experiment is not suitable for exact measurement, 

 for the capacity of the table when the cover is removed will not be 

 quite the same in the two cases, so that the gold leaves will not 

 get quite the same fraction of the charge. If a plate of india- 

 rubber or a plate of sulphur be used, similar effects are noticed; 

 the induced charge is always greater than with air, or, as 

 Faraday expressed it, the specific inductive capacity is greater. 

 We may give exact signification to this term in the following 

 definition : 



Specific inductive capacity. Let two condensers A and H 

 have exactly equal dimensions, and let the dielectric in A be air, 

 while in B it is some other substance. Then the ratio 



capacity of B 

 capacity of A 



is termed the specific inductive capacity of the dielectric in B. 

 It is usually denoted by K. 



The specific inductive capacity of a given specimen is probably 

 constant over a wide range of electric intensity, and assuming this 

 constancy, it is frequently termed the dielectric constant. It is 

 also termed the electric or electrostatic capacity of the material. 



If we consider the ease or difficulty of producing electric strain 

 in the dielectric, we obtain an analogy with elastic strain which 

 has some value. Suppose two equal condensers, A with air as 

 dielectric, B with a dielectric with constant K. If we charge A to 

 potential difference V, producing surface density cr and strain 

 D = <r, an equal potential difference in B will produce surface 

 density Kar and strain KD. To produce in B surface density <r 



and strain D, we only require potential difference = ' and the 



Jx 



