Northrup Millivolter Made under the Pyrovolter Patents of November 6, 1917 The new NORTHRUP MILLIVOLTER is a standardized portable deflection potentiometer, working on the Pyrovolter Principle. This instrument offers a convenient means for the accurate measurement of e.m.f.s. up to one volt, without the abstraction of any current from the source of the e.m.f. being measured. The MILLIVOLTER should generally be used in place of a millivoltmeter, where accuracy is of importance. It is valuable for taking the voltage drop in motors, for the measurement of low voltages over electrolytic baths where a carefully measured current is flowing, for use in thermocouple pyrometry, for measuring the internal resistance of dry cells, and for accurate measurement of large currents by reading the voltage drop over a low known resistance. The MILLIVOLTER has one hand-drawn scale and three ranges: 0-10 mv., 0-100 mv., and 0-1000 mv. It is described in Circular No. 9, which awaits your request. THE BROOKS' INDUCTOMETER In the Brooks' Inductometer is offered a compact form of variable inductance, with a self inductance range of 5 to 50 millihenrys, possessing the following advantages: 1. A fair degree of astaticism, which tends to eliminate errors due to stray field effects. 2. It is less expensive and at the same time fully as accurate as the AyrtonPerry instrument. 3. It occupies less space than the Aryton-Perry form. The instrument has a very nearly uniform scale, obtained by properly proportioning the coils. It may be used as a mutual inductance. It has a good ratio of maximum to minimum inductance (about 9 to 1) and also has as high a time constant as is consistent with good design and moderate size. The instrument is fully described in Bulletin No. 152, a copy of which will be sent upon request. THE LEEDS & NORTHRUP CO. ELECTRICAL MEASURING INSTRUMENTS 4921 STENTON AVENUE PHILADELPHIA Langmuir Condensation Pump Dr. Irving Langmuir (of the General Electric Co.) has developed an exceedingly interesting and valuable high-speed high-vacuum pump, and by special agreement with the makers, we are acting as sole distributors, for laboratory purposes. With this pump pressure as low as 10 bar have been obtained; and there is little doubt that very much lower pressures can be produced, by cooling the bulb to be exhausted, in liquid air, so as to decrease the rate at which gases escape from the walls. Some type of primary pump must be used; capable of developing a vacuum not less than 0.1-0.15 mm.of mercury. The illustration shows a Langmuir pump, connected to a two-stage primary oil pump-which is operated by a HP motor; all three parts of the outfit being assembled together on one base. JAMES G. BIDDLE 1211-13 ARCH STREET PHILADELPHIA If interested, write for copy of illustrated Bulletin 881, issued November, 1917; and also copy of paper by Dr. Langmuir. Laboratory Books McJunkin's Clinical Microscopy and Chemistry This work is really a clinical pathology, omitting no step from obtaining the Todd's Clinical Diagnosis Dr. Todd's work is more than a clinical diagnosis-it is a complete clinical 12mo of 687 pages, with 262 illustrations several in colors. By JAMES CAMPBELL TODD, M.D., Professor of Hill's Histology and Organography This manual for elementary students presents the fundamental facts clearly Mallory and Wright's Pathologic Technic This work gives the technic of bacteriologic methods, bacteriology of Octavo of 555 pages, with 181 illustrations. By FRANK BURR MALLORY, M.D., Associate Professor of Wells' Chemical Pathology The new matter includes amylase, Trichinella spiralis, anaphylatoxin, vita- W. B. SAUNDERS COMPANY Philadelphia and London SCHOOL AND SOCIETY It is in the interest of education that a journal such as SCHOOL AND SOCIETY, covering the whole field of education in the relation to the social order, should be widely read by those connected with institutions for higher education, on one side, and those connected with primary and secondary education on the other. Each should learn from all and the educational work of the nation should be conducted for the common welfare in the reconstruction period on which we are now entering. The difficult problems will require knowledge, wisdom and sympathy. A weekly journal of education, if properly conducted and generally read, can be of great service. The opinion in which SCHOOL AND SOCIETY is held is indicated by the following extract from an editorial article in the New York Evening Post. Two journals, the School Journal and the Teachers' Magazine, have been consolidated with SCHOOL AND SOCIETY. These have been both important publications in the educational field, the first having been founded in 1874, and the second in 1878. At one time the first attained a circulation of 50,000, and the second of 125,000. Perhaps one reason for the union with SCHOOL AND SOCIETY is indicated by the result of a post card referendum as to the five best educational journals for public libraries recently taken among all members of the National Society of College Teachers of Education, and the National Society for the Study of Education. Of 253 who replied, 221 recommended SCHOOL AND SOCIETY as one of the list, and the next most popular journal, the School Review, received only 179 votes. THE SCIENCE PRESS LANCASTER, PA. SCHOOL AND SOCIETY A weekly journal covering the whole field of Education in relation to the problems of American democracy. Published every Saturday. Annual Subscription, $3.00. Single numbers, 10 cts. SCIENCE A weekly journal devoted to the advancement of science. Published every Friday. Annual Subscription, $5.00. Single numbers, 15 cts. THE SCIENTIFIC MONTHLY An illustrated magazine, devoted to the diffusion of science, publishing articles by leading authorities in all departments of pure and applied science, including the applications of science to education and society. Annual Subscription, $3.00. Single numbers, 30 cents. THE AMERICAN NATURALIST A bi-monthly journal, established in 1867, devoted to the biological sciences with special reference to the factors of organic evolution. Annual Subscription, $4.00. Single Numbers, 40 cents. SCIENCE AND EDUCATION A series of volumes for the promotion of scientific research and educational progress. Volume I. The Foundations of Science By H. POINCARE. Containing the authorized English translation by George Bruce Halsted of Science and Hypothesis, ""The Value of Science," and "Science and Method." Price, $3.00 net. Volume II. Medical Research and Education By RICHARD MILLS PEARCE, WILLIAM H. WELCH, C. S. MINOT and other authors. Price, $3.00 net. Volume III. University Control By J. MCKEEN CATTELL and other authors. AMERICAN MEN OF SCIENCE A Biographical Directory containing sketches of about 5,500 leading American scientific workers, with a statistical study of American men of science. New edition in course of preparation. Price, $5 net. THE SCIENCE PRESS LANCASTER, PA., GARRISON, N. Y. Apparatus for Magnetic Precision Measurements The Fahy Permeameter Compensated Type Developed to meet the need for a reliable and convenient apparatus for the magnetic testing of single specimens (bars, rods, sheet material) and for the simultaneous comparison of two similar or dissimilar specimens. Fully described in Scientific Paper No. 306 of the U. S. Bureau of Standards. The Fahy Permeameter Simplex Type Represents a further distinct advance in the art of magnetic measurements. Specially designed to test single specimens only-two or more specimens can be compared, of course, by the results of consecutive tests-it fully retains the high accuracy and perfect consistency of the compensated apparatus, but gains considerably in simplicity of operation and speed of tests. The Magnet-Meter An instrument developed for the precise and rapid investigation of shaped permanent magnets, such as used in magnetos, electrical measuring instruments, etc. This instrument serves to determine not only the initial strength, but also the stability of magnets. Write for descriptive literature HERMAN A. HOLZ Apparatus of High Quality for Magnetic and Metropolitan Tower NEW YORK |