747,718. Photo-electric particle counter. MULLARD RADIO VALVE CO., Ltd. June 26, 1953 [July 8, 1952], No. 17206/52. Class 40 (3). [Also in Group XL (c)] In apparatus for sizing and counting the number of particles in a collection, the collection is scanned in a series of parallel lines by a flying spot which may be a spot of light or an electron beam means being provided for recognizing either the first or last scan of any particle so as to arrest the beam which then explores the particle to determine its size. General arrangement (Fig. 2). The particle is scanned by a cathode-ray tube raster or the beam of an electron microscope using a " guard spot " technique, i.e. effectively there are two beams, a main and a guard scanning adjacent lines. When one beam only intercepts the particle this is recognized as the commencement or finish of a particle and the scan is arrested and exploration of the particle takes place. The "guard spot" technique is described in detail in Specification 741,471. In the present instance a single spot is caused by a generator 10 to "wobble" from one scanning line to the other effectively forming two scanning lines. The spot wobbling potential is applied to a routing switch 11 the function of which is to recognize whether the pulse produced in photo-electric pick-up 12 by the interception of a beam by the particle is due to the main or guard beam and to direct the pulse to the appropriate channels M and G. The channels feed a first contact indicator 13 which is able to detect when the main beam only detects a particle and to pass a pulse to a switch control unit 14. The switch control unit changes over all of switches S3 to S8 shown in the Figure. Switch S3 transfers the spotwobble potential from the " Y " plates to the " X " plates, thereby imposing the wobble on the spot in the line direction of the raster; S4 transfers the output of routing switch 11 from first contact indicator 13 to correction indicator 15; S5 and S6 arrest the frame and line scans respectively; S7 and S8 render operative an " X " correction shift and a " Y " direction shift. The " Y " direction shift causes the spot to commence moving in the frame direction while, due to the transfer of the spot-wobble potential, wobbling in the line direction. When either the guard or main beam overlaps the edge of the particle the correction indicator receives one input from the routing switch 11 to indicate this and provide a voltage to the " X " correction shift which tends to move the beam on to the particle. This the beam follows the length of the particle until the correction indicator receives pulses from both beams. When this happens a particle completion circuit 18 fed from the correction indicator recognizes that examination of the particle is complete and resets switch control 14 which in turn resets all of switches S3 to S8. A condenser incorporated in the " y " direction shift 17 which has acquired an increasing charge as the exploration progresses down a particle is discharged through a counter and pulse amplitude analyser 19 to register the particle in its appropriate size group. The time bases recommence at the deflection they had when the spot. was arrested. Routing switch (Fig. 3). The output of the pick-up is applied simultaneously to the grids of a pair of valves V1, V2 which are also fed respectively on their suppressor grids with the spot-wobbling potential and the spot-wobbling potential inverted. Thus the output at the anode of V1 can be regarded as due to the main beam and that at the anode of V2 to the guard beam. The two outputs are applied according to the position of switch S4 to the "firstcontact indicator" or the " correction indicator ". First contact indicator (Fig. 4). The main beam output is differentiated and applied to a diode V3 which causes only the positive pulse produced on differentiation to be applied to the control grid of a normally blocked triode V4. This triode conducts, the resulting negative anode pulse cutting off valve V5 which forms with valve V6, a multivibrator. V6, normally cut off, now conducts. When the negative pulse produced by differentiating the main pulse appears it is applied to the suppressor of V6 blocking the valve and changing the multivibrator to its original state. Momentarily the entire anode current passes to the screen the output pulse there developed being fed to the switch unit. If, however, the guard beam also encounters a particle showing that this is not the first encounter with the particle a positive pulse is applied to the grid of a triode V7 which conducts and being connected in parallel with the grid resistor 20 of valve V6 reduces the potential of this grid. Hence the multivibrator cannot " flip " and no output pulse is produced. The output pulse developed by the main beam alone and applied to switch control unit 14 changes over switches S3 to S8. The switch control unit (Fig. 6, not shown), comprises a double triode cathode-coupled thermionic valve forming a bi-stable multivibrator. The pulse carried from the first contact indicator V5 changes it over. The pulse from the particle completion unit VI restores it. A relay connected in the anode circuit of one of the valves operates the switch contacts but electronic switches may be used the pulses developed at one of the anodes being used to operate them. The correction indicator (Fig. 8), is fed from the output of the routing switch (V.S.) and consists of a pair of series connected triodes V13, V14 the main output being applied to the grid of V13 and the guard output to the grid of V14. Thus the potential of the mid-point X rises and falls as one of the beams overlaps the edge of the particle during the exploration and this change is used to apply " X " correction to the " X " correction shift. The " X " correction shift (Fig. 9) comprises a condenser C to one pole of which the output of the time base is applied through a potential divider R1, R2, from which the supply to the " X " plate is taken. The output of the correction indicator (V.S.) is applied to a terminal of switch S7 through a resistor R3. With the switch made with the lower contact, capacitor C is short-circuited and the line time base operates normally. When the line time base is arrested by the operation of switch S6, switch S7 applies the shift potential to the " X " plate to maintain the spot on the particle. The " Y " direction shift (Fig. 10). The output of the frame time base is applied through potential divider R4, R5 to a condenser C1. The junction of resistors R4, R5 is corrected to the " Y " plates. Switch S8 is arranged to connect condenser C1 either to potential source B3 or to a pulse amplitude analyser (see Fig. 2). When the switch is in its lower position the time base operates normally. When the line time base is arrested however a battery B3 charges condenser C1 through resistor R6 continuing the " Y " deflection. The potential developed across condenser C1 is transmitted to the analyser when switch S8 is restored and is a measure of particle size. Particle completion unit.-This consists of a triode the grid and cathode of which are each connected to one of the anodes of the correction indicator (V.S.) so that an output from the triode anode is obtained only when both guard and main beams produce pulses simultaneously. The time bases each consist of a multivibrator of the Schmitt cathode-coupled type connected through a cathode follower valve to a Miller integrator. Switch S7 or S8 is inserted between the cathode of the cathode follower with its connection to the grid of the Miller integrator and the cathode lead of the follower. Opening of this switch arrests the time base and it continues to run at whatever point it left off when the switch is again closed. Specification 744,161 also is referred to.