326 SECTIONAL TRANSACTIONS.— A. 



Prof. W. M. Thornton. — Measurement of voltage by spark gaps. 



(i) The sphere gap as a voltmeter. Order of accuracy attainable. Differ- 

 ences of calibration, causes of differences. Ions in gap, spread of 

 field, corona on high tension system, oscillations on applied wave as 

 affecting peak value. Formulae proposed. Sphere gap must be 

 regarded as an empirical voltage indicator and not as a substandard. 



(2) Sparking in uniform fields between parallel plates with rolled edges. 

 This can be regarded as a standard voltmeter. It is fundamental, 

 consistent and accurate to more than 1 in 1,000 at all voltages. A 

 limiting gap for each diameter. This gap has no spread of field, 

 random ionisation in a much larger volume of air, irregularities are 

 eliminated. It is not affected by strong fields and can be used 

 without shielding. 



(3) Method of calibration. — Ellipsoid voltmeter found to be more reliable 

 standard. Capacitance divider method used as an additional check. 

 Order of accuracy obtained is individually 1 in 10,000 and is entirely 

 consistent. 



(4) Voltages of order of 100-200 kv. can now be measured to the same 

 degree of accuracy as the ampere and standard cell. 



(5) Measurement of impulse voltages. — The sphere gap has a serious time- 

 lag, which may give more to an impulse ratio of 6. (See Allibone, 

 J.I.E.E.) Theoretically the parallel plates have a negligible time- 

 lag, io" 8 sec. 



Mr. R. Davis. — The breakdown of dielectrics under transient electrical 

 stresses. 



The experimental study of the breakdown of gaseous, liquid, and solid 

 dielectrics has been greatly aided by the development of the impulse gene- 

 rator, and the high speed cathode ray oscillograph. With high voltages 

 some form of voltage divider for use in conjunction with the oscillograph is 

 required, and the precautions to be taken in using the resistor type are 

 discussed. Errors in recording are introduced chiefly through the capaci- 

 tance to earth of the high voltage arm of the divider. 



The breakdown of air is considered with reference to different electrode 

 systems, including the special case of a solid dielectric located between the 

 electrodes in parallel with the air path. Examples of the former are sphere 

 gaps, and of the latter, the insulators used in high voltage transmission 

 systems. Experimental laws are discussed, and the generalisation made, 

 that breakdown occurs most easily when the stress at the positive electrode 

 is greatest. The implications of this are considered in relation to polarity 

 effects in measuring gaps and flash-over of insulator systems. Reference is 

 made to the impulse strength of liquid and solid dielectrics, and, in the case 

 of the latter, to the part played by the immersing medium. 



Dr. S. Whitehead. — Some aspects of the electric strength of dielectrics. 



The general principles on which depend the ability of a gas to remain an 

 insulator under the application of a high voltage or electric stress are known, 

 but solid dielectrics present a more complex problem. Although the im- 

 mediate and best-known difficulties are those arising from heterogeneous 

 composition and structure, it is interesting to consider the ultimate limita- 

 tions which would apply even to a uniform dielectric. Firstly, there is the 

 liability of a dielectric to thermal instability which can be assessed by the 

 magnitude of the maximum voltage which can be applied, so as to produce 



