26 A
ANALYTICAL
CHEMISTRY
INSTRUMENTATION
Electrolytic Apparatus
be lodged in the winding interstices. Although noise may be objectionable in many cases, it is particularly serious when the pot is used in a high-gain amplifier circuit. Noise is best detected and estimated by observing the sharp pulses on a cathode ray oscillograph. Other
Rapid and Accurate Determination of Metals Electrolytic analysis of metallurgical
samples
demands highly accurate, yet easy to operate apparatus.
The Braun Model
PC
Electrolytic
Apparatus,
designed to meet every require-
ment For the most exacting technical analysis, meets every need for either routine or critical research work.
Uniform determinations of copper, lead, antimony, cadmium, nickel, tin, zinc and
other
metals can be made quickly, permitting continuous
routine
analysis
with
penditure of time and labor.
minimum
ex-
A l l units operate
independently of each other so that as many as
six
individual
simultaneously
on
analyses one
may
6-unit
be
made
outfit.
These
outfits, available in 2 , 4 and 6-unit assemblies, constitute the ideal equipment for busy laboratories.
For
complete
information
address
Dept. A-2.
BR AU N C O R P O R A T I O N 2260 East Fifteenth Street Braun-Knechl-Heimann-Co. San Francisco, Cahl.
Los Angeles, Caliio Scientific Supplies Co. Seattle, Washington
Conversion
Elements
Closely related to potentiometers is the class of devices in which dimensional changes can be used to cause a change in resistance. The resultant resistance change can be measured in a variety of ways and several techniques involve the simultaneous use of potentiometers. Wire strain gages are available in a wide variety of sizes and range. These converter elements consist of a resistance wire of small diameter which is stretched or compressed by the displacement under measurement. A change of length in the wire is accompanied by a change in cross section, both of which affect its resistance. Industrial units (Baldwin-Southwark) for this purpose consist of a grid pattern of wire mounted on a paper backing and fitted with connecting leads. In testing engineering structures, these units are cemented to the member under test. As a rule, the unit cannot be removed intact for future use. They are suitable for static as well as dynamic stresses and in the latter case will function satisfactorily at 100 kilocycles or more. They may be obtained with high or low temperature coefficients. When temperature effects must be compensated, a dummy gage can be exposed in the immediate vicinity but not subject to the stress under measurement. Conversely, such elements may be used to indicate temperature if they are cemented to a surface of known temperature coefficient of expansion, which is not subject to stresses of other origin. They are relatively inexpensive and are used in huge quantities. On occasion as many as one thousand units may be mounted on a structural unit undergoing stress analysis. The Statham gages produced by the Statham Laboratories, 8222 Beverly Blvd., Los Angeles, Calif., are precision units employing ribbon loops held by pins in an accurate mounting assembly. Safety stops prevent any stress beyond the limit of the gage ribbons, and the electrical connections are such that the change in resistance occurs in all four arms of a Wheatstone bridge. Relatively simple accessories are required in the form of dry cells and a panel-type microammeter. Applications include the measurement of velocity, acceleration, flow rate, and pressure. They arc also suited for dynamic measurements and conceivably for any phenomenon which can be converted to a small displacement of magnitude suited to the gage. For relatively large displacements certain forms of conducting rubber have shown promise as conversion elements. T o the best of our knowledge solutions of electrolytes have not been used extensively in these applications, although some organic liquids or mixtures have been found with high temperature coefficients. On the other hand, electrolytic tanks with a traveling probe have been used as potential dividing networks primarily in plotting equipotcntial lines. Thus in problems concerned with electron trajectories, such as the design of photomultiplier tubes, electron lenses, etc., it has been useful to supplement the complex and laborious calculations by setting up electrode models and tracing out the equipotential lines. Every point picked out by the measuring probe is transferred by a pantograph linkage to an adjoining sheet of graph paper, and complex contours may be deduced in a short time. Despite the inconveniences involved in electrolytic methods, they seem to present some advantages for complex systems which would require extensive complication if attempted with conventional voltage dividers.
