GEOLOGICAL SURVEY CIRCULAR 748

A Device for Automatic 

Photoelectric Control of the 

Analytical Gap for 

Emission Spectrographs
A Device for Automatic 

Photoelectric Control of the 

Analytical Gap for 

Emission Spectrographs

By John A. Dietrich, Elmo F. Cooley, 

and Kenneth J. Curry

GEOLOGICAL SURVEY CIRCULAR 748

1977
United States Department of the Interior

THOMAS S. KLEPPE, Secrefary

Geological Survey

V. E. McKelvey, Director

Free on application to Branch of Distribution, U.S. Geological Survey, 7200 South Eads Street, Arlington, VA 22202
CONTENTS

Page

Abstract ........................................................................... 1

Introduction ....................................................................... 1

Modification of arc stand for control device ........................................... 1

Mounting positioning motors ...................................................... 1

Beam splitter .................................................................... 1

Photocell unit .................................................................... 2

Photoelectric control device ......................................................... 2

Control unit ...................................................................... 3

Electronic circuit ................................................................. 3

Summary .......................................................................... 4

References cited .................................................................... 5

ILLUSTRATIONS

Page

FIGURE 1. Diagram showing positioning motor mounted ...................................................... 2

2. Photograph showing photoelectric control device mounted on a Jarrell-Ash arc stand ................ 2

3. Diagram of spectrographic optical system ........................................................ 3

4. Diagram of photocell unit installation ............................................................ 3

5. Schematic diagram of the electronic circuit....................................................... 4

TABLE

Page

TABLE 1. Electronic parts list ............................................................................. 4

in
A Device for Automatic Photoelectric

Control of the Analytical Gap for

Emission Spectrographs

By John A. Dietrich, Elmo F. Cooley, and Kenneth J. Curry

ABSTRACT

A photoelectric device has been built that automatically 

controls the analytical gap between electrodes during excitation period. The control device allows for precise control of the 

analytical gap during the arcing process of samples, resulting 

in better precision of analysis.

INTRODUCTION

Of the many factors that contribute to errors in 

spectrographic analysis, one is variation in the 

spacing of electrodes during the period of excitation. During excitation, part of each electrode is 

continuously burning away. In order to maintain a 

constant analytical gap, the operator must 

manually control the electrode spacing, a task 

which demands the operator's full attention. The 

total-energy method used by the U.S. Geological 

Survey for spectrographic analysis of geologic 

materials (Grimes and Marranzino, 1968) requires a burn time of 140 seconds for each sample. 

A photoelectric control maintains the correct gap 

spacing during this arcing of the sample.

A method for photoelectric control of the 

analytical gap has been described by J. D. Caylor 

(1967). Our technique differs from Caylor's primarily in that our device units are designed for installation on commercial arc stands with only 

minor modification of the stands. Our purpose was 

to devise a system that could be easily installed in 

the U.S. Geological Survey mobile spectrographic 

laboratories (Canney and others, 1957; and Arcs 

and Sparks, 1967), which provide rapid, on-thespot analyses for geologists in the field.

This paper describes the apparatus and its installation in a Jarrell-Ash Model 19-300 arc 

stand.

MODIFICATION OF ARC STAND FOR 

CONTROL DEVICE

MOUNTING POSITIONING MOTORS

The two positioning motors (Ml of table 1) may 

be mounted on the control rods in the front or 

back of the arc stand. In either case, the two 

motors' frames are attached to an aluminum plate 

to prevent their turning during operation. To 

mount the motors in the back of the arc stand, it is 

necessary to extend the horizontal control rods 

through the back of the arc stand (fig. 1). Onequarter-inch-diameter holes are drilled in the 

back panel and the support frame on line with the 

existing horizontal control rods. The extension 

rods are coupled to the existing horizontal control 

rods and to the positioning motors by steel sleeves 

which are fitted with set screws. A brass bushing 

is fastened to the support frame of the arc stand to 

give the extension rods added support and 

stability.

For front mounting of the positioning motors, no 

alterations are required other than replacing the 

control knobs with the positioning motors (fig. 2).

BEAM SPLITTER

The beam splitter is made by sandwiching a 

strip of solar reflective film between two 1- by 3- 

inch glass microscope slides. To project the 

electrode images onto the photocells, which are 

mounted in the photocell holder, the beam splitter 

is placed inside the arc stand near the front, perpendicular to the photocells, and bisecting the optical path between the back mirror and projection 

screen (fig. 3). A 21 /2-inch-diameter hole is cut in 

the side of the arc stand to allow projection of the
Back panel of arc stanck^

Aluminum plate

Positioning motor

(B

Set 

screws

Support frame

Vertical 

control

-frass bushing

tmw.)

M* MM) Extension control roa

Horizontal 

control rod

Back panel of arc stand

FIGURE 1. One of the positioning motors mounted on the back of the arc stand.

electrode images from the beam splitter to the 

photocells (fig. 4).

PHOTOCELL UNIT

The photocell holder (fig. 4) is made from micarta (laminated plastic). The dimensions of the 

holder are 3V4 by llb by PA inch. Four 31/64-inchdiameter holes are drilled side-by-side in a row in 

the holder to house the photocells. A 3/16-inchwide adjustment slot is cut in one end of the photocell holder to allow for minor vertical adjustments 

to correctly position the photocell unit. With the 

electrodes in proper positions, the holder is placed 

so that each electrode image fully covers one hole 

plus approximately half of an adjacent hole. The 

photocells are masked by a piece of cardboard attached to the inside of the photocell holder between the beam splitter and the photocells. A 1/8- 

inch-wide slit is cut in the cardboard the length of 

the four photocells and is covered with a blue 

acrylic plastic filter. The mask and filter prevent 

the cells from being affected by stray light and 

have a damping effect on the response of the 

electronic circuit.

FIGURE 2. Photoelectric control device mounted on the front 

of a Jarrell-Ash arc stand.

PHOTOELECTRIC CONTROL DEVICE

The photoelectric control device consists of a 

control box, two positioning motors, and four 

photocells mounted in their holder (fig. 2). The 

control box houses the power supply, electronic 

circuit (fig. 5), and control panels. The switches 

for operating the positioning motors are located on
BACK Beam splitter

(reflecting images

of electrodes)

Aluminum bar 

mounting bracket

FRONT H

FIGURE 3. Spectrograph optical system showing illumination path of the electrodes: A) electrodes; B) lens; C) front 

mirror; D) back mirror; E) beam splitter; F) opening in side 

of arc stand; G) photocell unit; H) projection screen.

the front control panel. Controls for the potentiometers in the bridge circuits are located on the 

back control panel. Electrical connections for the 

motors and photocells are also on the back control 

panel.

CONTROL UNIT

A three-position toggle switch, located on the 

front control panel, allows the controls to be operated manually or set for automatic control. The 

manual controls are used for loading, unloading, 

and positioning the electrodes. The electrode tips 

can be positioned at the desired arc gap by adjusting the potentiometers in the bridge circuits. 

Readjustment of the potentiometers is required 

only occasionally.

Photocell holder

FIGURE 4. Installation setup of photocell unit on the side of 

the arc stand.

To begin the arcing procedure, the images of the 

two electrodes are approximately positioned 

manually, and then the controls are set on 

automatic. Each electrode is controlled by its own 

electronic circuit. Only the action and control of 

one electrode will be described. During arcing of 

the sample, the electrode's tip is burned away, 

causing a lessening of the luminous image on one 

of the photocells. When one of the cells receives 

less light than the other cell receives, an output 

voltage is produced from the differential 

amplifier. This output voltage is connected to a 

power amplifier where the signal is amplified to 

drive a positioning motor mounted on the 

electrode control rod. The motor drives the control 

rod in the direction that will produce a balance in 

the bridge circuit and thereby maintain constant 

electrode spacing during the sample burning time.

ELECTRONIC CIRCUIT

The electronic circuit is a DC servo control 

amplifier. A diagram of the circuit is shown in 

figure 5 and a corresponding list of electronic
parts is given in table 1. The two electrodes have 

identical control circuits. The power supply is used 

for both circuits. Only one circuit will be described 

and illustrated.

Two photocells (one pair) are mounted in the 

image of one electrode. The pairs of photocells are

TABLE 1. Electronics parts list

Number Item Description

Rl Resistor .............. 2,200 ohms, 1/8 watt

R2 Potentiometer ........ 5,000 ohms

R3 Potentiometer, 10,000 ohms

miniature. 

R4, 5 Resistors ............. 10,000 ohms, 1/8 watt

PCI, 2 Photoconductive cells . GE type B425

IC1 Operational amplifier . Fairchild type UA741HC

Ql Transistor ............ GE 044C8

Q2 Transistor ............ GE 045C8

51 Switch ............... GC Electronics No.

35-728

52 Switch ............... SPST

Ml Gear motor ........... Globe Motors No.

319A126-2 

Dl, 2 Zener diode ........... 15 V 10 W

Power supply ......... Power One Model

HAA15-8

connected into opposite legs of a balanced bridge 

amplifier. A garden-variety, internally compensated, operational amplifier receives the signal 

from the bridge amplifier and provides a high-gain 

voltage output proportional to the small difference 

voltage created by any imbalance in the bridge. 

The signal from the operational amplifier is fed to 

a pole-sensitive power amplifier which drives the 

electrode positioning motor. This causes the motor 

to drive the electrode and its image in the direction required to cancel the difference in voltage 

seen across the input of the operational amplifier. 

Two diodes connected back-to-back across the 

motor protect the transistors from back EMF induced by the motor. A toggle switch enables 

manual operation of the motor drive. Power requirements of the system are plus and minus 15 

volts at 15 watts.

SUMMARY

The automatic gap control device described in 

this paper is simple and relatively inexpensive. 

Most arc stands can easily be modified to permit 

its installation. The device is needed by operators 

employing long-burn-time methods of

+ 15

FIGURE 5. Schematic diagram of the electronic circuit.
spectrographic analysis because it standardizes 

the arcing process and allows for precise control of 

the analytical gap between electrodes during arcing.

When using the total-energy method requiring 

a long burn time, as employed by the U.S. Geological Survey, the operator can prepare other samples for analysis during the arcing process. This 

allows for a 25 to 30 percent increase in the 

volume of spectrographic analyses. The device has 

been successfully used for over a year on both a 

Jarrell-Ash and an ARL arc stand.

REFERENCES CITED

Arcs and Sparks, 1967, United States Geological Survey lab on 

wheels: Ultra Carbon Corp., Arcs and Sparks, v. 12, no. 2, 

p. 3-5.

Canney, F. C., Myers, A. T., and Ward, F. N., 1957, A truckmounted spectrographic laboratory for use in geochemical 

exploration: Econ. Geology, v. 52, no. 3, p. 289-306.

Caylor, J. D., 1967, An automatic electrode loader and positioner for emission spectrographs: Union Carbide Corp. 

Y-12 Plant, Oak Ridge, Tenn., Report Y-1569, 12 p.

Grimes, D. J., and Marranzino, A. P., 1968, Direct-current arc 

and alternating-current spark emission spectrographic 

field methods for the semiquantitative analysis of geologic 

materials: U.S. Geol. Survey Circ. 591, 6 p.

AU.S. GOVERNMENT PRINTING OFFICE 1976-777-034/35