A Verstil Iistrumeit for the Meusurement

of Reflectuoce uand Truiismittuuce

IX6dk.4 _0DENSICHRO

No. 8830ADENSICHRON Patent No. 2424933

The Welch Densichron is an electronic densitometer of goodsensitivity and unusually high stability. It can be employedeither for quantitative measurement of optical density, or as asensing device for automatic control of various industrialprocesses in which constancy of transmitted or reflected lightis a criterion of the quality or uniformity of the product.

It consists essentially of a probe unit containing a photo-electric tube for detecting light energy and an amplifier towhich it is connected by a flexible cable. The amplifier in-corporates a milliammeter calibrated to indicate either opticaldensity on a "log" scale or percent transmission on a "linear"scale, depending upon the model.

The logarithmic optical-density scale extends from 0 to 1.5and density is readable to 0.01. A three-step range switch in-creases the range to a maximum density of 4.5 with no loss insensitivity or precision of readings. This range represents alinear response of the receiver to variations in light intensity ofover 30,000 to 1.

The meter movement used in the log-scale model of theamplifier is designed in such a manner that the deflections arevery nearly proportional to the logarithm of the reciprocal ofthe activating photoelectric current. This is advantageous inboth densitometry and photometry inasmuch as optical densityin both cases is this same function of photoelectric current andthe scale can therefore be calibrated in essentially uniformincrements of optical density. A lower scale on this meter isused for determining photographic exposure times.

The meter in the percent-transmission model uses a stand-ard meter movement and indicates percent transmission on alinear scale. This may also be considered as percent reflectanceor percent incident light upon the phototube. The full rangeof the scale for the first position of the range switch is from0 to 100% transmission. Other positions of the switch provideranges of 0 to 10%, 0 to 1% and 0 to 0.1%, respectively. Thelower scale on this meter is calibrated to read directly the opti-cal density corresponding to the percent transmission indicatedon the upper scale.

NTo. 8832A

REFLECTION UNIT

The amplifier also produces an A.C. output signal, thevoltage of which is a measure of the light intensity, and pro-vision is made for utilizing the direct current of the ammetercircuit in an external circuit. The instrument therefore suppliesa voltage which can be used to operate either a control mecha-nism or a standard ink recorder.

Principle of OperationThe outstanding feature of the Densichron is its patented

system for magnetically modulating the space current in aphototube.* This is accomplished by applying a 60-cycle alter-nating magnetic field across the path of the electron beam,thereby deflecting the electrons from their path to the anode 120times per second. The output current of the tube is thuschanged from its normal steady value to a pulsating output witha frequency of 120 cycles per second. When the usual direct-cur-rent output of a phototube is amplified, the measuring instru-ment is subject to severe zero drift necessitating frequent recali-bration, and its response is slow. The pulsating output from theDensichron phototube, however, is capable of stable, high-gainamplification with rapid response.

Although the Densichron can be used in a wide variety ofways it is a simple device to use and requires no special opera-tor skill. It will perform satisfactorily in routine operations forlong periods of time with little or no servicing, yet is readilyadaptable to innumerable types of special measuring techniques.

* Henry P. Kalmus and George 0. Striker, Rev. Sci. Inst.,19, 79 (1948).8880A. DENSICHRON WITH BLUE PROBE. Thisconsists of the amplifier with logarithmic-scale meter, blue-sensitive probe, metal probe support, five different measuringapertures, a cone with %-inch aperture, and a set of instruc-tions. The amplifier operates on 115 volts, 60-cycle A.C. only.except on special order. Each, $225.00

38830B. DENSICHRON WITH RED PROBE. This isidentical with No. 3830A except that a red-sensitive probe isfurnished in place of the blue-sensitive probe. Each, $225.00

W. M. WELCH SCIENTIFIC COMPANYDIVISION OF W. M. WELCH MANUFACTURING COMPANY

ESTABLISHED 18801515 Sedgwick Street, Dept. E Chicago 10, Illinois, U. S. A.

Manufacturers of Scientific Instruments and Laboratory Apparatus

4 FPEBRUARY 1955 9A

FUNDAMENTAL

FORMULAS OF PHYSICSEdited by DONALD H. MENZEL, HarvardUniversity

Here is a new authoritative handbook and pri-mary reference for all science majors who employthe methods or applications of mathematicalphysics.

Written by noted men in their fields, each of thethirty-one sections features the basic formulas usedin the major areas of modern science. Many ofthese formulas are heretofore unpublished.

Carefully chosen topics in the new handbook, suchas Physical Chemistry and Biophysics, providepractical discussions for all research workers. In

addition, extensive references to journals andmiscellaneous works guide the reader to particulardevelopments.

696 pages * 55/" x 83/8" * Published February 1955

GRIGNARD REACTIONS

OF NONMETALLIC

SUBSTANCESby M. S. KHARASCH and OTTO REIN-MUTH, University of Chicago

BRIEF DESCRIPTION: This book presents thefirst attempt at coordinated theoretical treatmentof Grignard reactions and represents the nearestthing to an exhaustive literature survey yetachieved. The preparations and properties of theGrignard reagents have been treated in great detail(Chaps. II-V) and the division of subject matterinto chapters has been made on the basis of co-

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1384 pages * 6" x 9" * Published October 1954

* a

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