EM8
The EM8 was the first commercialTransmission Electron Microscope produced by the company Carl Zeiss. It was developed in cooperation with the AEG, first designated the EM7 during prototyping. Both the EM7 and EM8 where developed based on the research of Brüche using Electrostatic Lenses exclusively, unlike the Siemens microsocpes of the time.
Design
The EM8, much like its predecessor is an Electrostatic focusing Microscope, relying solely on the potential of the cathode as well as, in later versions, the positive potential also provided by a second Cockroft Walton Cascade.
Electron Gun
Its electron gun is of the Steigerwald Telefocus Gun veriety which, unlike more contemparary gun designs places the crossover (point of convergence) not between the Wehnelt Cylinder and the Anode, but instead projects it far beyond the anode, usually in the aperture plane of the Objective lens. (presumably much like Köhler Illumination as used in the light microscope. This effectively eliminates the need for a condensor lens as is common in other microscopes of the time. However, unlike the magnetic microscopes of the time, this setup dose not allow the size of the illumination spot to be continuesly varies, instead it is constant. This likely lead to increased specimen heating and contamination buildup in comparison to the Siemens Übermikroskop and ÜM100.
Imaging System
Much like other microscopes of the time, the EM8 uses top entry cartridges for its specimens. These cartridges are placed into the stage by means of a crane mechansim incorperated into a semi automatic arilock. located between the Anode and Objective lens upper electrode.
The specimen is then focused (moved to the focal point of the Objective lens) by means of a mechanical lead screw, very unlike other microscopes of the time, which used the ability of the electromagnetic lens to change its focal length at will. This likely made focusing the microscope at high magnifications a rather fiddly affair, thus leading to the later addition of electrical means of fine focus (modulating a second voltage atop the High Voltage of the Objective lens (presumably)).
For low magnification, the projective lenses located some distance beneeth the objective, where switched off, thus only magnifying the sample by a factor goverened by the Lens Makers Equation and Magnification equation.
Lensmakers Equation: 1/f = 1/v - 1/u
f: Focal length of the lens u: Object Distance from lens plain v: Image Distance from lens plain
Magnification equation: m=v/u
m: Magnification factor u: Object Distance from lens plain v: Image Distance from lens plain
Thus, due to the small region in which the sample is focused onto the viewscreen / film, and the fixed focal length of the Objective lens, only the movement of the sample is viable as means of focusing. Thanks to the very small Aperture inhherent to uncorrected Electron Microscopes, this region becomes large enough that mechanical focusing becomes viable, all be it with increased difficulty at increasing magnificaitons. Later versions such as the EM8/IV allowed the focal length of the Objective to be varied slightly, thus allowing electric focus as can be seen by the above equations.
To further increase magnification, additional lenses, known as Projectives where used. These too where fixed focal length Electrostatic Lenses, which where stacked one atop the other. One having a large the other a small focal length. Thus powering either or changed the magnification. In addition to using one or the other, both could be activated simultaniously, allowing for the highest magnification the microscope was capable of. These lenses much like the objective where powered by the Cathode High Voltage Potential.
Acceleration Voltage
This reliance on a common high voltage used for both the acceleration of the electrons, as well as their focusing, gave the EM8 an advantage in terms of Stability requirments for the High Voltage System. Normally the focal length of a Electron Lens is dependen on the speed (voltage) of the electron beam they are are to focus. Since the focal length of Electrostatic lenses is dependent on the voltage ratio between the electron beam and its electrodes, one can easily surmise that any change in acceleration voltage would produce a change of focal length which is exactly opposit to the change that is espected by the change in acceleration potential. Thus allowing the microscope to be virtually immune to fluctuations of the Acceleration voltage.
As a result, the requirments on the high voltage power supply where far less then on Electromagnetic microscopes. This powersupply likewise was of a more modern design, using High Frequency AC to run the transformers as introduced by Vladimir Zworkin and James Hillier at RCA. This AC voltage was supplied by means of a Armstrong Osccilator. A second oscillator fed a HF isolation transformer which in turn supplied the heater voltage for the high voltage Diode tubes used in the Cockroft Walton cascade.
On later models, two such cascades where run from the main high voltage transformer, one supplying 60KV of positive potential, the other 60KV of negative potential.
Since stringent smoothing was not needed for the high voltage, no additional smoothing circuitry was used, only relying on the capactiance of the Cockroft Walten Generator to smooth out the voltage.
Camera
The microscopes Camera was likewise a rather interesting design, as it contained a magazine of film plates within the view chamber, which, when exposed, where automatically ejected by means of a airlock opperated from the same cam as the camera its self.
Thus every exposure could be directly taken out of the microscope without breaking the high vacuum there in, or requiring the removal of a separate exposed plate magazine as is common in other microscopes.
Development History
To our knowledge there seems to have been 3 to 4 versions of the EM8 throughout the years. It is our assumption that the version is designated by the addition of a "/version number" at the end of the EM8 model name. This however is not yet confirmed as we only have one example at the time of writing, which is designated as "EM8/IV" which lets us assume that this is the 4th version or revision to the microscopes design.
Fallowing is a list of the versions of the EM8 which are currently known to us, detailing their differences.
EM8
The original EM8 appears to have been a more refined version of the EM7 which preceeded it. It featured only 3 Electron Lenses, which are;
- Objective
- Projective I (High Mag)
- Projective II (Low Mag)
Focusing was done by purely mechanical means by a knob located in front of the stage.
EM8/II & EM/III
Since we do not know the differences between these two version we will consider them as one for the time being. The second / third revision of the EM8 introduced some design changes, including the addition of a Diffraction lens located beneath the Objective. This also gives the microscope even more magnification steps, of which only 3 are avalible at any given time, due to having to disconnect a high voltage plug (60KV) in order to change the magnification range.
Focusing was still (presumably) performed by purely mechanical means as in the previous model.
EM8/IV
Version 4, much like its predicessor also features a diffraction lens, in addition to the the ability to focus the objective by both mechanical (Corse focus) and Electric (fine focus) means.
This was accomplished, by means of changing the voltage going to the objective lens center electrode by having an auxilary powersupply which increases the cathode voltage by an amount settable via a potentiometer at low potential.
If other changes where implemented is not currently known as we (the Museum) have not yet begun restoration of a EM8/IV and only recently obtained its manual, but not its schematics.