410 PHYSICO-TECHNICAL INSTITUTION IN OHARLOTTENBUBG. 



published. The detailed accounts, however, of the most important 

 undertakings thus far completed arc contained in three quarto vol- 

 umes of investigations. Among those contained in the tirst two vol- 

 umes may be mentioned papers pertaining to thermometry and to 

 units of electrical resistance. 



The investigations in thermometry comprise such topics as the 

 influence .of the glass on the indications of the mercurial thermometer, 

 division of th(> thermometer, and determination of the errors of divi- 

 sion, determination of the coefficient of outer and iimer pressure, deter- 

 mination of the mean apparent coefficient of expansion of mercury 

 between i)^ C. and 1()(H C. in Jenagltiss, and investigations relating to 

 the comparison of mercurial thermometers. 



Four papers of exceptional \'alue relate to normal standards of 

 electrical r(\sistance. They ai'e the pro])able value of the ohm accoid- 

 ing to measun^ments made up to tne present time, the determination 

 of the (•alil)er correction for electrical resistance tubes, the normal 

 mercury standard ohm, and the normal wire standard ohm of the 

 lleichsanstalt. When one recalls that the ohm as a practical unit of 

 measurement is defined in terms of the resistance of a specified column 

 or thread of mercury, it will readily be seen that the work done at 

 Charlottenburg in this particuhir field is fundamental in character and 

 of the most universal imporUmce. , 



In passing it is worth}' of reiuiirk that all the standard resistances 

 designed and constructed at the Keichsanstalt are carefully compared 

 with the mercurial standards earl}'^ in each year. This custom is in 

 accordiuu-e with the action taken by the electrical standards connnittee 

 of t\u' J5ritish Association at Edinburgh in 181J2, when the mercurial 

 standard was definitely adopted. At this meeting of the connnittee 

 representatives of American, French, and German physicists (includ- 

 ing Von Helmholtz) were invited to sit as members. The methods 

 employed in these comparisons and the forms of the standards are 

 original with the Reichsanstalt. The new forms and methods admit 

 of a combined accuracy and convenience not previously attainetl. 



In addition to the work done in electrical resistance, the in\'estiga- 

 tion of the silver voltameter and the electromotive force of standard 

 Clark and Weston cells has been highh' productive of useful results 

 for the other two fundamental electrical measurements. Much remains 

 to be done in this latter direction, for the electromoti^'e force assigned 

 to the Clark and the Weston cell, even in the latest report of the 

 Reichsanstalt, is derived from neasurements by the silver voltameter, 

 while the electrochemical equivalent of silver is in doubt to a greater 

 extent than the electromotive force of the Clark cell. 



Perhaps the best indication of the valuable woi'k of the Reichsan- 

 stalt is to be found in the aiHuial "Th;ltigkelts])ericht.'"' This report 

 of the 3"ears' activity is published in the " Zeitschrif t fiir Instru- 



