Measurement of Small Capacities and Inductances. 507 



The measured capacity in micro-microfarads was found to 

 be 59° 9 5, the difference being about 12 per cent. In all 

 these cases, except tnat of the horizontal wire, the dif- 

 ferences between the observed and the calculated values 

 appear to depend upon the proximity of neighbouring- 

 objects or the ground, and upon the way in which connexion 

 is made to the commutatator. The capacity of a body 

 together with that of the connecting wire is not, strictly 

 speaking, the sum of the separate capacities of the body and 

 the wire measured in situ. In an experiment with one of 

 the iron strips, an apparent decrease of 5 per cent, in the 

 capacity was found when the connexion was changed from 

 the bottom to near the middle. The mathematical formula 

 gives us the value of the capacity of the body in infinite 

 space, but its measured capacity is in practice its capacity 

 relatively to earth and depends on two things : — the proximity 

 of neighbouring earth-connected bodies, and the manner of 

 attachment to the measuring device. The first always in- 

 creases the capacity above the calculated value, while the 

 second decreases it. It appears that the net result in the 

 case of the capacity of a disk of about five feet in diameter, 

 when insulated and hung up in the middle of a room 40 feet 

 long, 18 feet high, and 30 feet wide, with the connexion 

 taken away from the bottom edge, is numerically in excess of 

 its calculated or theoretical capacity in free space by about 

 10 or 12 per cent. 



Another interesting experiment was tried on the relative 

 capacity of two lengths of No. 30 copper wire, each 12 feet 

 long. One of them was preserved straight and suspended 

 vertically in the Laboratory as above described ; the other 

 was bent into a spiral about 2 inches in diameter, and 6* feet 

 in length over all, and then again into a closer spiral J inch 

 in diameter, and 18 inches in length over all. Taking the 

 capacity of the straight wire as unity, the capacity of the 

 long coil of large diameter was 0'8, and the capacity of the 

 short coil of small diameter was 0*32, showing how much 

 the capacity of a wire of given length is decreased by coiling 

 it into a spiral. 



One of the uses to which the above-described commutator 

 can be put is that of determining the capacity of leyden- 

 jars in absolute measure ; at any rate, the capacity cor- 

 responding to frequencies in the neighbourhood or 100. 

 Instrument-makers still retain the absurd custoinof speaking of 

 ley den-jars as pint, quart, and gallon sizes, instead of marking 

 on them their capacity in absolute measure. Now that leyden- 

 jars are so much used in wireless telegraphy, the necessity 



