LIGHTNING CURRENTS IN BURIED CABLE 291 



series resistance, R. When a constant voltage E is suddenly applied the 

 current through the coil is 



/ = I (1 - e-^'^'^^^) 



If the time constant L/R is small in comparison with the time to crest value 

 of a variable voltage to be measured, there will be no material delay be- 

 tween the crest value of the voltage and that of the current in the coil. 

 The applied voltages may therefore be obtained by multiplying the coil 

 current as obtained from the magnetic link reading by the series resistance, 

 provided the latter is much greater than the impedance of the circuit to 

 which the instrument is connected. Since the voltmeter in the present 

 case was designed to measure the voltage between the sheath and the cop- 

 per shield, and the surge impedance of this test circuit is less than 3 ohms, 

 comparatively low values of series resistance could be used. Three sepa- 

 rate solenoids and series resistances were used, to provide three voltage 

 ranges, from to 1 .5 kv, to 4 and to 9 kv. 



2.4 Magnetic Link Calibrations 



When several magnetic links, which have been exposed to the same field, 

 are inserted in the magnetic link meter, considerable differences in the de- 

 flections may be observed due to variations in the material of the links. 

 For this reason, all links used in this installation were placed in a magnetic 

 field of 257 gauss and were then classified accordmg to their response in the 

 magnetic link meter. This field was such as to produce deflections in the 

 most useful part of the meter range, centering around mid-scale. 



By this method the magnetic links used in the installation were divided 

 into four classes, in accordance with the ratio of the deflection observed on 

 the magnetic link meter for the link in question to the average deflection 

 for all links. The maximum deviation from the average in each class was 

 about ± 3 per cent. Instruments of the same t>T3e at all installations were 

 provided with links of the same class, to minimize inaccuracies. 



2.5 Observations of Incidence of Strokes to Ground 



To obtain data on the incidence of strokes to ground, observations were 

 made at one location within the test section, by means of an automatically 

 operated magnetic wire recorder arranged to record thunder picked up by a 

 microphone. The recorder was provided with a triggermg arrangement 

 which put it in operation on the approach of a thunderstorm, by action of 

 the voltage induced in an antenna by lightning current. The induced 

 voltage from a given lightning stroke was also made to record itself upon 

 the magnetic wire; and, from the delay between this indication and the 



