Electric Waves along Spiral Wires. . 429 
less effective inductance when used with the high frequency 
oscillations than when measured with alternating currents of 
low frequency. This was discovered to be due to the 
proximity of the turns of wire permitting a certain dielectric 
current to pass from turn to turn, or else a true conductor 
current to flow through the boxwood. 
Investigation showed that when employing a spiral wire 
as an inductance, measurements of the inductance at low 
frequency could not be considered as giving the true 
value of the inductance for high frequency oscillations. 
Even when the wire was wound as an open spiral with no 
solid dielectric as a core, there was still an uncertainty as to 
the true inductance. This led toa long series of experiments 
on the construction of small inductances of a form for which 
the inductance could be predetermined by calculation for 
high frequency oscillations. It was discovered that no form 
is so convenient as that of a long wire, bent into the form of 
a square with the ends brought out parallel at one corner and 
of the shape shown in fig. 4 (see Pl. XIII.). 
If 8 is the length of the side of the square in centimetres, 
and if d is the diameter of the wire of which it is made, then 
the inductance L of the square in centimetres is given by the 
equation 
168 : 
L=8s {2-303 logis —26 } 
If the tails of parallel wire by which the current is brought 
to the square have a length /, and are at a distance D centi- 
metres apart, then these create an additional inductance L’, 
where 
(ee 24 4-606 log + 5} : 
Hence the total inductance is L+ L'. Five squares of wire 
were accordingly constructed of copper wire 2 millimetres in 
diameter. The sides of these squares in centimetres and feet 
and inches, and calculated inductances, were as follows :— 
Square. Length of side. Calculated Inductance. 
1} 0 al Bae eye» $245 ems. —27 ft. =56,000 ems. 
INO e pagccateae AD. eee A Tt. SPATE EY ae 
INGE Sos. ees. 200.5, 1 = ft: 3 ins. =a Nn 
PHOS As So cath 168 3 (=o ft. (sins. =1/9.274)\ }; 
INO HD waap due 132i) ih =4 ft. 4 ams, == fey (0) 
