US3222942A

US3222942A - Device for detaching mercury drops from a capillary tube

Info

Publication number: US3222942A
Application number: US81466A
Authority: US
Inventors: Davis Herbert Macdonald; Lockwood William Hartshorn
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1960-01-16
Filing date: 1961-01-09
Publication date: 1965-12-14
Anticipated expiration: 1982-12-14
Also published as: CH389281A; GB897037A; DE1448259B1

Description

Dec. 14, 1965 H. M DONALD DAVIS ETAL 3,222,942

DEVICE FOR DETACHING MERCURY DROPS FROM A CAPILLARY TUBE Filed Jan. 9. 1961 FIG.- 1.

United States Patent 3,222,942 DEVICE FOR DETACHING MERCURY DROPS FROM A CAPILLARY TUBE Herbert MacDonald Davis, Barnehurst, and William Hartshorn Lockwood, Pinner, England, assignors to United Kingdom Atomic Energy Authority, London, England Filed Jan. 9, 1961, Ser. No. 81,466 Claims priority, application Great Britain, Jan. 16, 1960, 1,655/ 60 4 Claims. (Cl. 74102) This invention relates to devices for detaching mercury drops from the end of a capillary tube. Such devices are used in polarography to detach the drops from dropping mercury electrodes. The present invention has particular application in differential polarography, in which it is important that the mercury drops from two such electrodes should be detached as nearly simultaneously as possible, and that the period between successive drops should be as constant as possible.

According to the present invention a device for detaching mercury drops from a capillary tube comprises a moveable mounting for the tube, stop means for limiting the movement of the mounting between two positions, an electromagnet having a moveable armature, the armature being loaded to return to an unenergised position, a lost motion coupling between the armature and the mounting for moving the mounting from one position to the other when the electromagnet is energised, and means for restoring the mounting to said one position.

The moveable mounting may comprise an arm pivoted about a substantially vertical axis, one end of the arm being adapted to hold a capillary tube in a substantially vertical position and the other end being located between two stops, the lost-motion coupling being between the armature and said other end of the arm and the restoring means comprising spring-loading means acting on said other end.

The lost-motion coupling may comprise a pin connected to the armature and an apertured member con nected to the arm, the pin passing through the aperture and having a head adapted to engage the member, the head being spaced apart from the member when the armature is in its unenergised position. The springloaded means acting on said other end of the arm may comprise two springs acting in opposite directions.

The electromagnet may comprise a solenoid, the armature being a spring-loaded rod arranged to move axially within the solenoid. The electromagnet may comprise a core portion coaxial with the armature having a nonmagnetic end-portion which abuts the armature when the armature is drawn into the coil.

Said one end of the arm may be bifurcated into two branches each adapted to hold a capillary tube.

To enable the present invention to be more readily understood, attention is directed, by way of example, to the accompanying drawings, wherein:

FIG. 1 shows a part sectional plan view and FIG. 2 shows an elevation of an embodiment of the invention.

Referring to FIG. 1, an arm 1 pivoted about a roller bearing 2 mounted on a bracket 7, is bifurcated at one end into two branches 3 and 4, each of which carries a clamp 5 whose jaws are recessed at 6 to hold a capillary tube (not shown) in a vertical position. The mercury is fed to each capillary tube from a separate reservoir. The other end of the arm is located between adjustable screw stops 8 and 9 which limit the movement of the arm about the bearing 2. The stops are mounted on an angle-section base-plate 10 which is secured to the bracket 7 by bolts which pass through a vertical polarograph stand 11, and also serve to secure the device to the stand. As

3,222,942 Patented Dec. 14, 1965 an indication of the dimensions, the base-plate is 6 inches long and the drawings are to scale.

In the quiescent state, the position of the arm 1 is such that center stabling

springs

29 and 30 are in their natural condition (both lightly compressed to the same extent). The return stop 8 is adjusted to barely touch the arm 1, and the forward stop 9 is adjusted to allow the arm 1 to undergo a small movement.

Fastened to the arm 1 is a bracket 12 comprising a fork-like vertical member 13, through the aperture in which passes a pin 14 having a head 15 adapted to engage the member 13. The pin 14 is adjustably fastened to a soft iron rod 16 constituting an armature, which slides in a brass tube 17 on part of which is wound a solenoid 18. The solenoid forms part of an electromagnet comprising a tubular soft iron housing 19 having an end-cap 20, and a threaded core-portion 21 which is a screw fit in the end-cap 20 and is secured by a lock-nut 22. On the inner end of the core-portion 21 is a small brass projection 23 which abuts the end of the armature 16 when the latter is drawn into the solenoid.

In the unenergised state the position of the armature 16 is determined by spring-loading means comprising a coil spring 24 (not shown in FIG. 1) which encircles the armature and holds a washer 25 on the end of the armature 16 in contact with a bracket 26 having a vertical forked portion 27 through which passes the pin 14. The bracket 26 is fastened to a clamp 28 which secures the electromagnet to the base-plate 10. The bracket 26 is made of steel and also helps to complete the magnetic circuit.

In the quiescent state, i.e. with the electromagnet unenergised, the arm 1 is barely touching the return stop 8 and the armature 16 is held against the bracket 26 as described. The device is so adjusted that in this state the head 15 of pin 14 is spaced apart from the apertured member 13 of bracket 12 (as shown in FIG. 1), so that a lost-motion coupling is provided between the armature and the arm, i.e. the coupling is such that motion of the armature from its unenergised position is not communicated to the arm until the armature has moved some distance. As a result of this lost motion, when the electromagnet is energised the armature has acquired considerable momentum before head 15 engages member 13; thereafter member 13 travels with head 15, and the arm 1 is moved rapidly to a position against the forward stop 9. The resulting inertial and viscous forces acting in shear detach the mercury drops from the two capillary tubes. When finally adjusted, the clearance between head 15 and member 13 in the quiescent state is about 0.25 inch and the arm movement between

stops

8 and 9 is less than 0.1 mm. (exaggerated in the drawing), the latter distance being adjusted to the minimum which gives reliable detachment of the drops.

The projection 23 helps to prevent the armature being held in by residual magnetism when the electromagnet is de-energised. Sufiicient clearance is provided between the armature 16 and the tube 17 to prevent air-damping during inward movement of the armature. The arm 1 is made of aluminium alloy to reduce its inertia.

The center stabling

springs

29 and 30 restore the arm 1 to its quiescent position against return stop 8 without a sudden movement which might detach a second mercury drop. The also prevent rebound of the arm.

The solenoid has 550 turns of 24 standard wire gauge (0.022 inch diameter) copper wire and is designed to be energized by 12 volt pulses of 0.02 second duration, obtained from the differential polarograph circuit.

In one such polarograph the period between successive pulses is 7 seconds, the levels of the mercury reservoirs being adjusted to give both capillary tubes approximately the same natural dropping period (i.e. by gravity alone) in the range 1012 seconds. The two tubes are about 4 /2 inches long and are selected to have matched bore diameters in the range '.O5 -0.1 mm.

The precise timing of the present dropping device results from the lost-motion coupling between the armature and the arm; together with the short travel of the arm. When a voltage is appliedto an inductive circuit the current rises exponentially with time, so that the'magnetic force acting on the armature also rises. from zero. A rigidcoupling WOllldlhBlBfOlG result in the capillary tubes being moved with'the low initialvelocityof the armature, leading to imprecision in the dropping action] In the present device the armature acquires considerable momentum before movement is communicated to the arm, and the' capillary tubes. travel at a very high velocity over a very short: distance, thus: defining closely the instantiof'detachment. The small movement of the capillary tubes also'avoid disturbing the solutions in which theyare immersed, Precision of the timingbetween suecessive drops is also improved by keeping the time-constant (L/R) of the solenoid as lowas practicable, in the present case about 0.01 second, so' that the force on' the armature rises rapidly after the/initiation of'the current pulse to the solenoid.

We claim:

1. A device for detaching mercury drops from a capillary tube comprising a moveable'arm including a mounting for holding a capillary tube, an electromagnet'having armature moveable between an unenergized and an energized'position, a lost-motion coupling coupling said armature to said arm, thecoupling servingto delay move ment of the arm until after the armature has moved a predeterminedamount,first loading means actingon said armature to return it to the unenergized position, two substantially non-resilientistops defining the limits of the movement of saidarrn,andsecond-loading means acting a vertical position, means definingzaverticalaxis about.

which said arm.is pivoted, anelectromagnethavingan. armature moveablevbetween an'unenergized and an-energized position,- a lost-motion coupling. couplingsaid arma-- ture to said arm, thetcoupling serving-to delaymovement of the arm until after the armature has moved a predeterminedamount, first loading means acting on said'armature to return it to the unenergized position, two substantially non-resilient stops defining the limits of the movement of said arm, andsecond loading means acting on said arm to. restore said arm to-one of said limits after being moved therefrom tothe other of said limits by the energization of said electromagnet.

3. A device for detaching mercury drops from a vertically mounted capillary tube" comprising. a moveable arm; means to secure a capillary tube to said armadjacent one endof the arm and inavertic'al position; means defining a vertical axis about which said arm is pivoted;

an electromagnet having an armature moveable between an unenergized and an energized position; an apertured member secured to said arm, a pin secured to said armature; the pin having-a head and passing through the aperture in said member, the head beingspaced apart from said' member when said armature is in the'unenergized position and moveable onenergization oi the electromagnet to engage themember so that a lost-motion coupling is-provided to transfer motion from said armature to said armyfirstleading means acting on said-armature toreturn it to the unenergized position; two substantially nonresilient stops defining the limits of the movement of said arm; and second loading means acting on said arm to restore said arm to one ofsaid limits after being-moved therefrom to the other of said limits by the energization of said electromagnet.

4. A device in accordance with claim-3' wherein said second loading means comprises a pair of'center stabling helical springs acting on the other end'of said arm.

References Cited bylthe Examiner UNITED STATES PATENTS 905,136 12/1908' Barnum 317186 X 2,361,295 10/1944 Kanner et al. 324--3l 2,63 8,804 5/ 1953 Heinrich. 2,689,330 9/1954 Staunton 3243l 3,049,939 8/1962 Beard 74-526 BROUGHTON G; DURHAM, Primary Examiner.

SAMUEL BERNSTEIN, FREDERICK M. STRADER,

Examiners;

Claims

1. A DEVICE FOR DETACHING MERCURY DROPS FROM A CAPILLARY TUBE COMPRISING A MOVEABLE ARM INCLUDING A MOUNTING FOR HOLDING A CAPILLARY TUBE, AN ELECTROMAGNET HAVING ARMATURE MOVEABLE BETWEEN AN UNENERGIZED AND AN ENERGIZED POSITION, A LOST-MOTION COUPLING COUPLING SAID ARMATURE TO SAID ARM, THE COUPLING SERVING TO DELAY MOVEMENT OF THE ARM UNTIL AFTER THE ARMATURE HAS MOVED A PREDETERMINED AMOUNT, FIRST LOADING MEANS ACTING ON SAID ARMATURE TO RETURN IT TO THE UNENERGIZED POSITION, TWO SUBSTANTIALLY NON-RESILIENT STOPS DEFINING THE LIMITS OF THE MOVEMENT OF SAID ARM, AND SECOND LOADING MEANS ACTING ON SAID ARM TO RESTORE SAID ARM TO ONE OF SAID LIMITS AFTER BEING MOVED THEREFROM TO THE OTHER OF SAID LIMITS BY THE ENERGIZATION OF SAID ELECTROMAGNET.