Convenience in PH Measurements The Type K Potentiometer Outfit shown in the accompanying illustration has come into very general use for the measurement of hydrogen ion concentrations. Ample sensitivity and accuracy are retained under a great variety of experimental conditions, at no sacrifice of convenience. No painstaking adjustment of parts is required. In our Catalogue S75 are described and illustrated various outfits for measurement of hydrogen ion concentrations, with various degrees of accuracy, from the potential divider method using a voltmeter (Hildebrand) to the Type K Precision Potentiometer Outfit with a highly sensitive galvanometer. Much information that may be valuable to you-thirty pages of it-is also included in this new publication. Catalogue S75 will be sent you if, in writing, you will mention your position and particular interests, so that we may also send you those of our future publications which may be useful to you. LEEDS & NORTHRUP COMPANY ELECTRICAL MEASURING INSTRUMENTS 4901 STENTON AVENUE PHILADELPHIA, PA. Leitz Microscopes are the Standard of the World MICROSCOPE STAND "AA" It is especially recommended for Research Investigations and Micro Photography. For this purpose stand "AA" has been selected as standard by the leading laboratories of the U. S. Government, the Hospitals, Universities and Industrial Institutions throughout this and other countries. The mechanical construction is of utmost rigidity and every part of its adjustment of thorough precision. Leitz Apochromatic Objectives are individual in Quality and their Optical Efficiency embodies the very latest achievements in optical science, as developed by the Leitz Research Department. The Leitz Apochrom9ts excel in Range of numerical aperature Color correction For service that requires exceeedingly "fine" and uniform current regulation, we offer the following rheostats: Price on application 60 blocks" thick 22" long 8 wide *This rheostat is supplied with three movable terminal plates, as shown in the picture. JAMES G. BIDDLE CADMIUM STANDARD CELLS For use with potentiometers, electrometers, and ballistic galvanometers, or wherever else a precision standard of electromotive force is needed. Negligible Temperature Coefficient. Contents hermetically sealed in glass so that lowering of electromotive force due to evaporation is impossible. Mounted in moulded box, or unmounted. My comparison certificate accompanies each cell without charge. On November 13, 1916, three normal cadmium cells made by me were certified by the Bureau of Standards as differing from their group of reference cells by less than one part in one hundred thousand, all three having the same value. On March 30 1920, after the lapse of three years and four months, the same three cells were again compared by the Bureau of Standards. The maximum difference among the three was less than one part per hundred thousand, and the maximum difference from their certified value of 1916, was less than one part per hundred thousand. In other words, these cells showed a degree of constancy to all intents equal to that considered most highly satisfactory, if not remarkable, in the very best type of precision resistance standards. (see Bul. Bur. of Stand. Vol. 5, No. 3. p. 427.) The cells in question were in nowise special. They were made of the materials regularly prepared by me, and were set up with no greater care than that constantly employed in the making of my standards. Marion Eppley, (A.M., Ph.D.) Physico-Chemical Apparatus. NEWPORT, R. I. |