Measurement of Small Capacities and Inductances. 499 



the dimensions of which are 18 feet high, 32 feet wide, and 

 44 feet long. 



Some preliminary experiments in the open air showed that 

 the capacity of parallel wires suspended in the air, and 

 insulated, is by no means equal to the sum of their separate 

 capacities when in free space, even when the wires are not 

 very near together. A series or experiments was therefore 

 carried out in the Pender Laboratory with flat iron strips, to 

 investigate this fact more carefully. Eleven lengths of iron 

 strip one inch wide, the thickness being O05 inch, and 

 15 feet 4 inches long, were suspended vertically from the 

 ceiling of the Laboratory by silk strings passing over por- 

 celain buttons. To keep the strips straight they were fastened 

 to the floor through pieces of ebonite. Connexion was made 

 at the bottom end of each strip by a terminal to a straight 

 bare copper wire connected to the above described commutator, 

 so that the capacity of any one strip or of any number of 

 strips in parallel could be measured. In the first place, the 

 individual capacity of each strip taken alone and isolated, 

 was measured and found to be nearly the same in each case. 

 Hence in the tabulated results, the absolute values of capacity 

 are not given, but the capacity of each of the above strips by 

 itself in the room, and held vertically, is taken as unity. The 

 voltage of the battery employed in charging the strips was 

 about 148 volts, and the number of revolutions of the com- 

 mutator 1200 per minute, corresponding to a frequency of 

 80. One terminal of the insulated battery used for charging 

 was connected to the earth, and also one end of the galvano- 

 meter. The other ends of the galvanometer and battery 

 were connected to the outside brushes on the commutator, the 

 middle brush being connected to the insulated strip or strips. 

 A series of capacity measurements was made, taking the 

 strips one at a time, two at a time, three at a time, &c, and 

 at various distances apart, viz. : 12, 6, and 3 inches, and 

 also when close together. The results are tabulated in the 

 following table (p. 500) . 



The last column in the Table gives the figures showing 

 the sum of the individual capacities of the strips, and it will 

 be seen that the total measured capacity of n strips taken 

 together at a distance d inches apart is always very much 

 less than the sum of the individual capacities of the n strips. 

 that is, much less than n times the capacity of one strip, and 

 the figures show that as the strips are brought nearer together 

 this difference increases. When the strips are about (> inches 

 apart, that is to say, separated by a distance equal to about 

 3 per cent, of their length, the capacity of the strips in 



