142 
the conductor carrying the lightning surge are used to 
increase the accuracy, particularly where currents of 
opposite polarity may flow. These links will measure 
only the maximum peak in a multiple stroke. 
Fulchronographs [17], devices where a number of 
links are mounted on a rotating wheel, permit deter- 
mination of the variation of current with time. The 
resolution for continuing currents is good. The high- 
speed wheels used permit separation of amplitudes of 
successive high current peaks, but the speed is insuffi- 
cient to determine the wave shapes of current peaks. 
Magnetic links applied in circuits in which induct- 
ances or capacitances are used in combination with 
resistance permit determination of the maximum rate 
of current change of a current peak. 
The photographic surge-current recorder [10] meas- 
ures the intensity of light produced by a surge current 
across a short spark gap. A range of 0.1 amp to 150,000 
amp is claimed for this device. 
The cathode-ray oscillograph [3] is the most versatile, 
but also the most elaborate device for measuring surges. 
Several types of oscillograph must be used to record 
the high-speed (current peaks) and slow-speed (con- 
tinuing currents) components of a complete lightning 
stroke. The development of the sealed-in cathode-ray 
tube has made this instrument simpler and subject to 
automatic operation. 
Of considerable value for determining the mechanism 
of the discharge are the so-called Boys cameras [5, 15]. 
With high-speed rotating film, or high-speed rotating 
lenses, the speed of propagation of the leaders and 
return strokes can be analyzed. The low-speed cameras 
give valuable information on the time sequence of 
current peaks and the continuing-current flow between 
multiple discharges. It has been possible to obtain fairly 
good correlation between density of the film exposed to 
a stroke and the current flow causing the illumination, 
principally for the continuing components. A micro- 
photometer has been used to correlate these quantities. 
For accuracy it is necessary to avoid overexposure of 
the film, as well as to have highest sensitivity for the 
weaker illumination. For this purpose multiple-lens 
cameras are used with apertures varied to cover the 
complete range of exposure expected in a straight-line 
relation. 
Electric field measurements permit investigation of 
lightning-stroke phenomena from the point of view of 
the charges involved. Some of the measurements made 
differentiate the leader, the return stroke, and the 
continuing-discharge portions of strokes. The wave 
shapes of atmospherics and correlation with various 
forms of lightning discharges have been investigated. 
Such measurements have been extended to cover dis- 
tances of hundreds of miles. By using two or more 
instruments, the distance of the source of atmospherics 
can be determined with good accuracy. 
A rather useful means of determining lightning char- 
acteristics is the examination of damage produced by 
lightning. By reproducing similar effects in the labora- 
tory it is possible to determine approximately the 
type of stroke responsible for the damage, as well as 
ATMOSPHERIC ELECTRICITY 
the amplitude of current peaks and the charge con- 
ducted in the stroke. Damage produced on metal parts 
of airplanes is in many respects the only means by 
which lightning currents occurring in the clouds can be 
determined. 
Lightning Protection 
The statistical knowledge gained over the past fifteen 
to twenty years has largely confirmed the effectiveness 
of a properly installed lightning-rod system (advocated 
by Benjamin Franklin more than 150 years ago) to 
protect ordinary buildings and houses against direct 
strokes. Such systems consist of interconnected air 
terminals mounted on the highest points of the struc- 
ture, and connected to the ground at several points. 
The ‘Code for Protection Against Lightning” issued 
by the National Bureau of Standards and designated as 
Handbook H40, and the British Code of Practice C. P. 
1:1943 published by the British Standard Institution, 
contain the essential rules to follow for imstallation, 
materials, size, and other factors essential for an ef- 
fective durable installation. 
For continuity of electrical service to homes, farms, 
and factories, many practices have evolved, based di- 
rectly on the many lightning studies and their results. 
For protection of electrical apparatus, the lightning 
arrester is a commonly accepted device. Such arresters 
have the property of conducting surge currents to 
ground at a reduced voltage. Any system power current 
(follow current) which may flow through the arrester 
immediately following the lhghtning discharge is in- 
terrupted, and the line is restored to its original con- 
dition. Many different types of arresters are in use. 
In some cases plain gaps are used for protection. These 
cannot limit the voltage applied to the apparatus to 
voltages as low as arresters. After sparkover of the gap 
occurs, circuit power current usually follows and must 
be interrupted by opening a breaker. 
Transmission lines of higher voltage ratmgs can be 
effectively protected by the use of ground wires prop- 
erly suspended above the transmission wires to inter- 
cept the direct strokes. The lightning currents contact- 
ing the ground wire and the towers to which they are 
connected will raise the tower potential by virtue of 
the resistance of the tower to ground and the current 
in the stroke. To this is added the inductive voltage 
drop caused by the inductance of wire and tower and 
the rate of rise of current on the current front. By 
reducing the ground resistance to less than one ohm 
per 12 kv of circuit voltage, it has been possible to 
reduce outages on circuits of 66 kv and above to an 
extremely low value. Ground resistance can be reduced 
by the use of buried wires—counterpoise—or deeply 
driven ground rods. 
Lower voltage circuits have been made lightning- 
resistant in many cases by the use of wood as insulation 
in addition to the normal porcelain or glass insulators. 
Reclosing circuit breakers are another tool for pre- 
venting circuit outages. These breakers are able to open 
a circuit and reclose it in as little as one-fifth of a second 
by means of suitable relaying. Proper balance between 
