28 A
ANALYTICAL
VITRE
VITREOSIL
SIL
CHEMISTRY
INSTRUMENTATION
quency measurements. It is primarily suited to operate into a 1-ma. 1400-ohm recorder and its characteristics as a d.c. amplifier are rated on a load resistance of this value.
Standard f^atporatory Apparatus and Utensiis
disc true
Include crucibles economy
these
porous representing
Precipitates can be filtered, ignited and w e i g h e d all in the same utensil w i t h ease, assuring accuracy w h i l e saving time and
handling. The VITREOSIL porous discs are fused to the crucibles and entirely obviate the use of filter papers. A l l discs and crucibles have the same outstanding properties of fused silica and can withstand ignition temperatures up to 1,000°C In addition, there is a range of sizes in three standard shapes, and four grades of porosity are available for retention of material ranging from coarse grains to fine precipitates.
The THERMAL SYNDICATE, LTD. 12 East 46th St. · New York 17, N.Y. VITREOSIL • TRANSLUCENT
LABORATORY
WARE
· TRANSPARENT
Wide Band
Amplifier
An amplifier for very different purposes is now available from the Hewlett-Packard Co., 395 Page Mill Road, Palo Alto, Calif. The new Model 460 A wide band amplifier is suitable for amplifying pulses as short as 0.01 microsecond. It is characterized by a rise time of approximately 0.0026 microsecond without overshoot. This novel instrument has been described by Ginzton, Hewlett et al. [Proc. Inst. Radio Engrs., 36, 956 (1948)]. It is based upon the principle of the distributed amplifier, wherein conventional amplifier tubes can be connected in parallel, so that their plate currents add but their capacities do not. As a result, the band width can be increased beyond the point where the individual tubes have a gain of 1, which is the limiting point in conventional cascaded stages. The tube capacities are used as the shunt capacities of simple low pass filters or artificial transmission lines and by proper termination and balance of the grid and plate lines, currents from all tubes can arrive at the output terminals in phase. Enough tubes are used in each stage to produce a gain of 2.72 and then such stages are cascaded to produce the required gain. The wide band property and extremely fast response are particularly useful in such applications as the photomultiplier tube scintillation counters. Saturable
Reactors
We continue to be corrected on our statements about the availability of saturable reactors and shall gratefully pass on the information to readers who may be interested. One of the most useful corrections comes from R. L. Burwell of Northwestern University, who not only informs us that the Electron Mfg. Co., 4596 North Elston Ave., Chicago, 111., supplies a 40watt reactor which saturates on 5 ma. but also describes a temperature-control device employing the reactor. We wish to recommend reading the article by Burwell, Peterson, and Rathmann [Rev. Sci. Instruments, 19, 608 (1948)], which tells how temperature control to ±0.0025° can be accomplished in the range of 25° to 210° C. A Thermistor is used as the sensing element in a Wheatstone bridge. The amplified unbalanced signal, which is phase-sensitive, is rectified and applied to the control winding of the saturable reactor and thus regulates the heating current in proportion to the deviation. The successful results achieved in this investigation seem to answer another point which has been raised repeatedly and for which we do not have any extensive information. That is the degree of reproducibility of Thermistors. Apparently, they are reproducible to within a few thousandths of a degree under the conditions described in this paper. While this writer has no desire to excuse his shortcomings by pleading the exactions of his job, it is becoming increasingly evident that one cannot keep track of all that is known to be useful. I t is therefore extremely helpful when a correspondent sends a reference, a correction, or a bit of information which we can pass on to others. We are doubly certain that our readers are grateful for this assistance and effective supplement to the brief remarks which can be made monthly about some of the many advances in instrumental techniques.
