April 27, 1876] 



NATURE 



519 



If, for instance, it is known that with a certain battery j 

 power and a resistance of 100,000 units we have a deflec- 

 tion-measure of 100, it is deduced, when with an unknown 

 resistance and the same battery power the deflection of 50 

 is obtained, that the resistance must be twice as great 

 (namely, 200,000), since the observed effect is halved. 

 This system is that generally pursued, but, like the 

 other measurements upon submerged cables, comes under 

 the effect of earth-currents ; and to meet this contin- 

 gency Sir William Thomson has arranged a new method, 

 bearing upon the principle that the insulation of a cable i 

 may be determined from the proportion of loss (during a | 

 given time) of electric power that has been imparted to i 

 it. In the following description it will be seen that this i 

 loss is measured by the deflection due to the current ' 

 entering the cable to make up the loss, and this deflection | 

 is compared with another deflection obtained by altering 

 suddenly by a small quantity the battery power em- 

 ployed. The latter deflection being a measure of a known 

 force or potential, the other measure for lost potential 

 is determined, and consequently the loss of potential 

 known. 



" The cable being offered to me again from midnight 

 till 2 A.M. on the 17th, I made,"' says Sir W. Thomson, 

 " another series of tests at that time for the main object of 

 measuring the insulation-resistance. I found the line in 

 a much less disturbed state, and was able to make a per- 

 fectly satisfactory insulation test by the ordinary galvano- 

 meter method. I applied, however, also a new method 

 which (no electrometer being available) I had planned to 

 meet the contingency of the line being disturbed by 

 earth-currents so much as to render the ordinary test 

 unsatisfactory, but not so much as to vitiate an electro- 

 meter-test. This method, which I think may be found 

 generally useful for testing submerged cables when an 

 electrometer is not available, is as follows : — i. Apply the 

 ordinary test by battery and galvanometer for a certain 

 time. 2. Insulate the cable for a certain time and then 

 shunt the galvanometer to prepare for No. 3 (unless you 

 have conveniently available a second galvanometer suit- 

 able for discharges). 3. Instantaneously reapply the 

 battery, through the insulation galvanometer properly 

 shunted (or a special discharge galvanometer), to the 

 cable, and observe the maximum of the sudden deflection 

 produced. 4. Go on repeating Nos. i, 1, and 3 as long 

 as you think proper, according to circumstances. 5. To 

 determine the proper ballistic constant of the galvano- 

 meter for utilising the observed result of No. 3, tind the 

 maximum of the sudden deflection which takes place 

 when a sudden change of electrification is produced by 

 instantaneously changing by a small measured difference 

 the potential of one electrode of the galvanometer, the 

 other electrode being in connection with the cable. 6. 

 The change of potential which, in the operation of No. 5, 

 would give the same deflection as that observed in No. 3, 

 is equal to the change of potential which the conductor 

 of the cable has experienced during the time when it was 

 left insulated according to No. 2. Hence calculate the 

 insulation-resistance in ohms or megohms as in the ordi- 

 nary electrometer method when the electrostatic capacity 

 of the cable is known." 



In carrying out this test, the 20-cell insulation battery 

 (with its poles joined through 20,000 Siemens' units) was 

 applied, zinc to cable, through the insulation galvanometer 

 with a shunt of 5,000 Siemens' units on it. Then, the 

 galvanometer indication was read and recorded every ten 

 seconds for three and a half minutes, when the cable was 

 insulated during a minute according to No. 2 of the 

 directions above, and a shunt of 30 substituted for the 

 5,000. At the end of the minute the battery was instan- 

 taneously reapplied, the throw of the galvanometer ob- 

 served according to No. 3 and the shunt of 30 removed, 

 and 5,000 reapplied. The cable was again insulated for 

 a minute, the galvanometer shunted with 50 (instead of 



30 used the first time), and the operation of No. 3 re- 

 peated. The proper ballistic constant of the galvanometer 

 was determined by applying alternately full power and \% 

 of full power of the insulation battery ; the change from 

 one power to the other being made in each case as 

 instantaneously as possible. Twelve galvanometer read- 

 ings taken at ten seconds' intervals during the second 

 and third minutes of the electrification gave for mean 

 deflection 127, and the readings taken from the fourth to 

 the twenty-fourth minutes gave for mean deflection 82' i. 

 The sensibility of the galvanometer in the condition in 

 which it was used for these readings was such that a 

 deflection of 290 would have been given by the actual 

 battery, with a resistance of 1,000,000 Siemens' units. 

 Hence the insulation-resistances proved by the mean 

 observed deflections were for the deflection, 127 fro.xi the 

 second and third minutes 2,280,000 Siemens units, and 

 for the mean deflection 82*1 from the fourth to twenty- 

 fourth minutes 3,540,000 units. The new method de- 

 scribed gave, as the mean of the observed ballistic 

 deflections or '' throws," the number 89'8, or say 90. 

 The ballistic deflection due to instantaneously changing 

 the potential by ^q of that of the insulation battery, in 

 accordance with the rule of one to five above, was found 

 to be 112 divisions. This is 1:^ time the preceding mean 

 throw (90), which therefore showed a change of potential 

 equal to ^V of that of the battery. Hence the mean fall 

 of potential was ^^ during the minute, or at the rate of 

 sooTT P^^ second. The capacity of the cable (measured 

 in the way presently described) had been found to be 991 

 microfarads. Hence the insulation-resistance is ^^~ , or 

 3-027 megohms, or 3,170,000 Siemens' units, corresponding 

 to the 3,540,000 units given by the ordinary method. 

 With copper to line, a fresh series of tests gave 3,520,000 

 megohms, or 3,690,000 Siemens' units. 



In the reduction of the insulation-resistance of the 

 whole cable to its insulation-resistance per knot, it has 

 to be observed that as the insulation of the cable is in- 

 versely as its length, one knot of the cable will give an 

 insulation resistance equal to that of the whole cable 

 multiplied by the number of knots' length in the whole 

 cable. 



Measuretnent of " Capacity. ^^ — Just as the chemist has 

 his vessels for measuring out quantities of liquid, so has 

 the electrician his special arrangements for measuring out 

 quantities of electricity ; but there the analogy ends, for 

 while the measure of the liquid is direct and visible, the 

 electrician infers his measured quantity generally by the 

 mechanical work effected on the index of the measuring 

 instrument, or by the absence of such work. The appa- 

 ratus used in practice for measuring quantities of elec- 

 tricity is termed a " condenser." " Condensers " are 

 constructed having any required capacity, and if such a 

 condenser of which the capacity is known is charged 

 from a battery, then discharged through the measuring 

 instrument, and the deflection produced noted, it is only 

 required to charge from the same battery the cable or any 

 other condenser of which the capacity is to be measured, 

 then to note this discharge deflection, and by proportion 

 to deduce the unknown capacity. On short lengths of 

 cable this procedure is actually adopted, but on long 

 lengths it becomes liable to error, chiefly from the fact 

 that as with long lengths some perceptible time is re- 

 quired to discharge the cable, the ballistic throw or 

 sudden deflection produced upon the measuring instru- 

 ment by the rush of electricity from the cable does not 

 measure all that passes out. It is consequently necessary 

 to devise some method Uke the following used by Sir W. 

 Thomson, in which the charge from the cable (communi- 

 cated thereto by a diff'erent battery power to that charging 

 the condenser, but the relative powers being known) is 

 neutralised by a charge of opposite electricity from the 

 condenser, and the neutralisation declared by the non- 

 production of movement in the measuring instrument. 



