January, 1922. 



SCIENTIFIC AGRICULTURE. 



151 



cause of failure of liofhtniug rods and the 

 kind of material, type of rod, and system 

 of installation necessary to perfect pro- 

 tection. 



The discharge of a static machine, as the 

 electrical machine with its Leyden jars is 

 now universally called, may be either 

 direct or alternating, depending on the 

 resistance of the conductor in relation to 

 the charge. If the resistance is great 

 enough to dissipate the energy in one 

 passage of the current, there will be a 

 single impulse in one direction, but if not 

 then the charge will oscillate back and 

 forth — an alternating current — until 

 the energy is turned into heat. An alter- 

 nating current builds up high self induc- 

 tion, which although obstructing the 

 current, does not aid in dissipating the 

 energy. However, the self-induction has 

 a tendency to cause the current to side- 

 flash to other conductors in the vicinity, 

 although the resistance of the wire car- 

 rying it may be only a small fraction 

 of an ohm. This was clearly proven by 

 Lodge thirty years ago in one of his 

 classic experiments with alternating 

 discharge from Leyden jars. To gain an 

 idea of the relative importance .of self- 

 induction and resistance Lodge computed 

 these for various frequencies ranging from 

 12 million to a quarter of a million per 

 second. "At the higher frequencies self- 

 induction alone counts. At a quarter mil- 

 lion vibi-ations the material just begins to 

 matter, and iron has a trace more im- 

 pedence than copper", (page 116, Light- 

 ning Conductors and Lightning Guards). 

 "Impedence'' is a term used to include 

 both resistance and self-induction. Be- 

 lieving the frequency to be a million or 

 more, he therefore fovoured iron. 



Today, however, we know that lightning 

 is not alternating but direct, although 

 there may be several discharges in rapid 

 succession along the same path but all in 

 the same direction. This was proven by 

 De Blois in 1913, by means of an oscillo- 

 graph in measuring actual lightning 

 flashes. No oscillations were found, but 

 rather undirectional discharges, the du- 

 ration of which ranged from .0002 to 

 .0016 seconds, the average being more 

 than one hundred and fifty times as slow 

 as the quarter million frequency alluded 

 to l)v Lodge. Hence at the now known 



duration of lightning discharges, especial- 

 ly the slower ones, resistance is an import- 

 ant factor, and from scientific considera- 

 tions the lower the resistance of the light- 

 ning conductor, t^ie better the results that 

 may be expected. This is borne out in 

 practice. Of the rodded buildings burned 

 by lightning and which have come to the 

 writer's attention during eigliteen years' 

 study of the prol)lem, all but two have 

 either been rodded with iron rods or iron 

 centred ones, both of which exceptions 

 were badly installed. Therefore so far 

 as Canadian practice is concerned it may 

 be taken as finally established that copper 

 is the most desirable element for use in 

 protection against lightning. . 



In this result the question of durability 

 played an important part. Jron rods are 

 short-lived where subject to extreme con- 

 ditions, often being found corroded off 

 at the ground. 



It is sometimes stated that a building 

 imperfectly rodded is in greater danger 

 than if not protected at all. This does not 

 appear to be borne out either by scientific 

 considerations or bv the practical data col- 

 lected at the 0. A . C. 



Since it is very important that the resist- 

 ance in the lightning rod circuit should be 

 as low as possible, and since usually the 

 ground resistance is many times as great 

 as that of the conductor itself, it naturally 

 follows that great pains should be taken to 

 sink the cable to permanent moisture and 

 give it as much earth contact as possible. 

 If a water pipe or gas main is near at 

 hand the grounding can usually be ma- 

 terially improved by connecting the cable 

 to the pipe: generally the resistance of 

 water and gas pipe groundings does not 

 exceed 5 to 7 ohms., W/hile resistance be- 

 tween cable (8 to 10 feet) and soil may 

 run from 25-50 ohms in ordinary soils to 

 100 ohms or more in sandy, gravelly or 

 filled soils. 



From the time current induction was 

 discovered by Faraday the practice of 

 connecting to the lightning rod various 

 metallic parts of the structure lying in its 

 vicinity became more general. Lodge set 

 up experiments to study whether this was 

 good practice. While lie found it possible 

 even with all neighboring metals connect- 

 ed, to produce sparks from the conductor 



