DEPARTMENT OF TERRESTRIAL MAGNETISM. 239 



on page 235 and on page 236 (eclipse observations). Assistance was 

 received in this work from Messrs. D. M. Wise, M. B. Smith, and C. 

 A. Kotterman. 



With reference to the photographic records of the potential gradient 

 and the electric conductivity of the atmosphere, it was found that 

 some of the records obtained show strikingly the effect of the potential 

 gradient on the recorded conducti\dty; this is especially true during 

 periods of large negative potential-gradient when the recorded positive 

 conductivity frequently became vanishingly small. It is hoped soon 

 to make a special study of this matter and also of the relation between 

 the atmospheric ions of ordinary mobility and those of much smaller 

 mobiUties. This latter study is held in abeyance pending reconstruc- 

 tion of certain parts of the apparatus in use. Some time has been 

 devoted to consideration of various types of batteries for use with 

 the atmospheric-electric equipment; at present batteries composed 

 of silver-chloride cells are being tried; thus far the results appear quite 

 favorable. 



An abstract of a paper by Dr. Mauchly, on the results of his earth- 

 current experiments, is given on pages 263-265. 



A preUminary test of the special Toepfer planimeter for scaling of 

 observatory traces was carried out and reported upon by C. A. Kotter- 

 man, laboratory assistant. It was found that while a high degree of 

 accuracy was obtainable with the planimeter, the time required for 

 determining mean areas by this method was excessive and would be 

 hardly warranted in actual practice except for limited work requiring 

 very precise measurements. It will probably, therefore, be best to 

 arrange for scaling of the mean areas of traces obtained in the observa- 

 tory work by the use of transparent glass plates with suitable area- 

 scales etched or engraved on the under sides; this method of scaling 

 is now used at the observatories of the United States Coast and Geo- 

 detic Survey and elsewhere. 



STANDARDIZING AND INSTRUMENT WORK. 



Reference to standardizations of instruments and determination of 

 constants has been made on page 237. The amount of this work was 

 greatly increased over that of previous years because of the return of 

 field parties and of the Carnegie. Simultaneous comparisons of instru- 

 ments to determine constants and to determine corrections on our 

 adopted magnetic standards were made for 12 magnetometers, 5 dip 

 circles, 4 earth inductors, 2 universal magnetometers, 1 sea deflector, 

 1 marine collimating-compass, and for the compasses of 5 dip circles. 



Investigations relating to the sources of error in terrestrial-magnetic 

 measurements and the causes for changes of instrumental constants 

 were continued as far as time and personnel permitted. The com- 

 pilations of data and the investigations of possible changes in the dis- 



