Measurement of Small Capacities and Inductances. 501 



top, and in nearly close contact at the bottom. Such an 

 arrangement of insulated wires was found to have a capacity 

 of 2685 micro-microfarads ; in other words, about thirteen 

 times that of one wire, so that in this case the square-root 

 law also holds very closely. In the same way the capacity 

 of four parallel vertical wires 111 feet long, and one-tenth of 

 an inch in diameter, arranged vertically at the corners of a 

 square of six feet in the side, was measured, and was found 

 to be 583 micro-microfarads ; in other words, about two and 

 a half times the capacity of one single wire of the same 

 kind in the same position. Similar measurements have 

 shown that 25 wires 200 feet long, arranged with their 

 bottom ends close together, and about ten feet above the 

 ground, and their top ends about two feet apart, had a 

 capacity not greater than five times that of one single wire 

 of the same kind in the same position. Hence it is clear that 

 in bunching together or placing in contiguity a number of 

 vertical wires in the air so as to form a radiator for use in 

 Hertzian Wave Wireless Telegraphy, account must be taken 

 of this fact, and the assumption must not be made that the 

 capacity of n parallel wires placed vertically in the air and 

 insulated, even approaches a value equal to the sum of their 

 individual capacities, unless the distance between the wires is 

 a large percentage of their length. 



This effect was further examined in another series of 

 experiments made in the Laboratory. Two of the above- 

 mentioned iron strips, suspended and insulated as before, 

 were placed at different distances apart, and their united 

 capacity measured and compared with the mean value of 

 their separate capacities. Taking this mean A*alue of the 

 capacity of a single strip as unity throughout, the results are 

 as follows : — 



Table II. 



Horizontal distance 



Horizontal distance 



Capacity of the two 



between strips in 



between strips as a 



strips together, that 



inches. Strips 174 



percentage of their 



of one strip being 



inches long. 



length. 



unity. 



90 



49 



1-81 



57 



31 



L-85 



30 



14 



1-70 



17 



9 



L-57 



13 



7 



L-60 



4 



2-1 



i -<°> i 



H 



09 



L • 1 5 



