17 
ELECTRICAL WAVES AND SOME OF ITS APPLICATIONS. 
Resistance of Iron for High Frequency Discharges. 
The rapid decay of the oscillations when iron wires formed the discharge circuit 
has been already noted. This has been observed by Trowbridge (‘Phil. Mag.,’ 
December, 1891), who found, by photographing the spark, that there was evidence 
of much fewer oscillations when iron wires were used instead of copjier. 
For very rapid oscillations the resistance R' is given by R' = s/^pjxhK (see Lord 
Rayleigh, “ On the Self-Induction and Resistance of Straight Conductors,” ‘ Phil. 
Mag.,’ 1886), where R is the resistance of the wire for steady currents, I the length 
of the wire, g the permeability, and p = 2Trn, where n is the number of oscillations 
per second. 
Since the expression involves g, we should expect the resistance to be much 
greater for iron wires than for wires of the same conductivity, but non-magnetic. 
To determine the resistance of iron wires a very simple method was used. The 
fall of deflection, due to the detector needle, arranged as in (fig. 2) was noted. The 
iron wire was then removed and a copper one of the same diameter substituted. 
Since the inductance of the circuit was practically unchanged, if the damping in the 
two circuits are equal, the resistances should be the same. A short piece of high- 
resistance platinoid wire was introduced into the circuit of the copper wire, and the 
length adjusted until the deflection was the same as in the first case. When this is 
so, the resistance of the platinoid wire, together with the resistance of the copper 
wire, is equal to the resistance of the iron wires for the frequency employed. 
The resistance of the copper wires was calculated for the frequency used, but was, 
in general, small compared with the resistance introduced. 
I'he resistance of the platinoid wire was also calculated, but was found to be 
practically the same as for steady currents. The length of wire placed in the current 
was 265 centims. ; spark length was ’25 centim. 
Kind of wire. 
Diameter. 
R. 
R^. 
R^/R. 
Soft iron. 
centims. 
•025 
61 
II-8 
1 
i 
1-9 1 
?? . 
•047 
2-62 
12-8 
4-9 i 
j 
•094 
•57 
9-2 
16 i 
I )) . 
•295 
•051 
4-2 
72 
1 Pianoforte steel wire . 
•062 
1-51 
11-3 
6-5 
Nickel wire. 
■062 
•66 
3-7 
5 9 
In the above Table, R is the resistance for steady currents, Rj the resistance for a 
frequency 1‘6 millims. per second. The last column gives the ratio Rj/R for the 
different wires. In the case of the wire ‘295 centim. in diameter, the resistance is 
72 tunes the resistance for steady currents, while in the case of the wire of 
•025 centim. diameter, it is only 1’2 times, 
MDCCCXCVII.—a, d 
