In the case of thin-walled tubes when the compensation arrangement of type D is not feasible, other compensating tricks 2 · 3 are applicable.

R E F E R E N C E S

1. NONINDUCTIVE WATER-COOLED STANDARD R E -SISTANCES, C. C. Paterson, E. H. Rayner. Journal, Institution of Electrical Engineers (Londont Eng-land), volume 42, 1909, page 455.

2. A STUDY OF THE INDUCTANCE OF FOUR-TER-MINAL RESISTANCE STANDARDS, F. B. Silsbee. Bulletin, United States Bureau of Standards, volume 13, 1916, page 375.

3. NOTES ON THE DESIGN OF FOUR-TERMINAL RESISTANCE STANDARDS, F. B. Silsbee. Journal of Research, United States Bureau of Standards, volume 4, 1930, page 73.

4. HEAVY SURGE CURRENTS—GENERATION AND MEASUREMENT, P. L. Bellaschi. AIEE TRANSAC-TIONS, volume 53, 1934, January section, pages 80-94.

5. Discussion by Theodore Brownlee of HEAVY SURGE CURRENTS—GENERATION AND MEASURE-MENT. AIEE TRANSACTIONS, volume 53, 1934, March section, pages 481-2.

A n Automatic Oscillograph Wi th a Memory

Discussion and author's closure of paper 46-67 by A . M . Zarem, presented at the AIEE winter convention, New York, N . Y., January 2 1 - 2 5 , 1946, and published in AIEE T R A N S A C T I O N S , 1946, March section, pages 150 -4 .

C. M. Foust (General Electric Company, Schenectady, N. Y. ) : The author has adopted what seems to be the best of several methods of recording wave shapes of random surges, particularly those of very short time duration. In general three methods have been used for this work: the time delay cable, the moving magnetic tape or links, and the after glow or memory of the fluorescent screen of a cathode-ray oscillo-graph. Invariably the conditions under which the recordings have to be made will determine which of these methods is most appropriate.

The first cathode-ray memory oscillo-graph was developed by A. W. Hull of the General Electric research laboratory, and reported to the National Academy of Sciences at its autumn meeting on No-vember 18, 1935.1

Prior to this, early in 1935, Doctor Hull initiated the building of a 3-tube os-cillograph, which was designed and con-structed in the general engineering labora-tory of the General Electric Company under the writer's supervision. This in-strument was completed in 1936, and ap-plied successfully.2

Possibly the most important difference between this oscillograph and the one de-scribed by Mr. Zarem is in the beam accelerating voltage. The 1936 instrument utilized a beam voltage of 15 kv, achieving therefrom an oscillograph writing speed substantially in excess of that obtainable with the author's 4,500-volt beam. Writ-ing speeds in excess of one meter per micro-second are realized readily at 15 kv on single-sweep basis.

The successful use of this type oscillo-

graph resulted in a second instrument being built along the same lines, so that six cir-cuits could be studied simultaneously. This second memory oscillograph was com-pleted in 1943.

C. C. Herskind (General Electric Company, Schenectady, N. Y.) : The memory oscillo-graph is a very useful and valuable tool for the development and testing of mercury arc rectifiers. I t has been used for a num-ber of years for this purpose.

We have had two memory oscillographs in use for a number of years in connection with work on large mercury arc rectifiers. Each of these instruments has three cathode-ray tubes. By operating the instruments together it is possible to obtain records on all six anodes of a rectifier at one time.

The type of information which the memory oscillograph provides is illustrated by the following experience. During the years 1942-43 some of the ignitron rectifiers on the large electrochemical installations were operating with too high frequency of arcback. Extensive factory and field tests to determine the cause of these arc-backs were made without success.. Finally a memory oscillograph was installed on one of the faulty units. I t was connected to record the anode-to-cathode voltage. Records showed that the arcbacks were occurring throughout the inverse cycle with almost a uniform distribution. Of par-ticular interest were records of arcback occurring so late in the inverse cycle that no circuit breakers tripped or interruptions occurred, and arcback would not have been suspected without the memory oscillograph record. The uniform pattern of arcback occurrence greatly reduced the probable causes of arcback.

The cause of arcback was found to be the flow of mercury from the exhaust pipe line into the rectifier striking the anode. The time of arcback from such a cause would be expected to be uniform in dis-tribution throughout the cycle and, there-fore, in agreement with the data obtained by the memory oscillograph. Once the cause was known, the fault was readily corrected.

The foregoing example illustrates an important application of the memory oscillograph. I t undoubtedly will prove valuable in other fields where random phenomena are experienced and continuous records would be impractical.

A. M. Zarem: C. M. Foust and C. C. Herskind separately point out the previous use of oscillographs of the memory type in the study of the operation of mercury arc converters. Both men kindly have offered specific examples of the utility of such an instrument as a research tool. I should like, however, to summarize here the essential differences between the ordi-nary memory oscillograph and the auto-matic memory oscillograph. This matter is touched upon briefly in the first and last paragraphs of the paper under discussion.

The automatic camera and the electronic control unit form the heart of the auto-matic memory oscillograph. The combi-nation of these two components with a beam blanking device and a cathode-ray oscillograph puts the resulting instrument

in a class of its own. In addition to obtain-ing memory time, continuous monitoring of electric systems then becomes possible.

The advantages of automatic operation are tremendous when electrical conditions preceding randomly occurring transients are to be investigated. As the time of occurrence of these transients is unknown, the use of an operator to change films after a record has been obtained is not very practical. Under many conditions of opera-tion, the recording equipment may not be a t ground potential, making frequent film changing activity hazardous. In addi-tion, since there is no assurance that a sporadic transient of a given type will occur twice, there are even instances where vital information may be lest during the time delay between changing of films.

Several other features serve to enhance the utility of the automatic memory oscillograph. Perhaps the most important of these is the inclusion of means for exer-cising accurate control in presetting the memory time desired. Other essential additions are the control cf the instant of beam blanking, means for adjusting the camera exposure time, and the inclusion of a clock mechanism for identifying each transient with its time of occurrence.

The use of additional cathode-ray tubes for studying multiple phenomena is obvious. In this connection mention should be made of the new multiple beam cathode-ray tubes now commercially available. These tubes employ two sets of insulated accelerat-ing guns and deflection plates, allowing the simultaneous appearance on one cathode-ray screen of two wave forms from sources at different potentials. Two very slow transients also can be studied with one cathode-ray tube by means of electronic switching.

I t is believed that the automatic memory oscillograph is a definite advance in the development of a suitable device for re-cording electrical conditions preceding, during, or after the occurrence of an electric transient. I t already has proved its value both as a laboratory tool and as a practical instrument for monitoring electric systems.

Thermal-Demand-Meter Testing Techniques

Discussion of paper 46-69 by E. E. Lynch and M . E. Douglass, presented at the AIEE winter convention, New York, N . Y., January 21 - 2 5 , 1 9 4 6 , and published in AIEE TRANS-A C T I O N S , 1946, March section, pages 124-8 .

Perry A. Borden (The Bristol Company, Waterbury, Conn.): Without the late Doc-tor Paul Martyn Lincoln—father of the thermal watt demand meter—discussion of a paper dealing with that instrument seems almost a sacrilege. If he were here at the present time, or if he is permitted to be with us in spirit, we can appreciate his satisfaction in knowing that after 30 years of an up-hill fight the thermal meter has come into its own as a means for establishing the demand of an electric power load. Upon first read-ing the paper by Lynch and Douglass, I

514 Discussions AIEE TRANSACTIONS