ELECTKOiN MlCKOSCOl'Y 



uniform thinning is to be attained. By plot- 

 ting the equipotentials, Fisher (29) has found 

 the apjM-opriato shape of the anode, placed 

 at a gi\(Mi distance from a pointed cathode, 

 necessary to gi\'e uniform current density 

 over the anode surface. The anode must be 

 pear-shaped at its periphery, with a flat 

 central portion. Such a configuration could 

 be achieved by placing suitable washers on 

 either side of a flat sheet. The specimen 

 perforates almost simultaneously at a nvnn- 

 ber of points in the flat area. Polishing must 

 be stopped as soon as the first holes appear 

 in order to maintain a uniform current-den- 

 sity distribution and to stop rounding of the 

 edges. This technique, which seems the most 

 promising of those reviewed above, has so 

 far been applied to stainless steel, iron, gold, 

 and iron-cobalt. It can obviously be ex- 

 tended to other metals and alloys. 



Electrolytic Jet Machining. The tech- 

 niques for obtaining thin foils from bulk ma- 

 terial described above are limited to mate- 

 rials that can be obtained in the form of thin 

 sheets <200 ^ thick. The other criticism 

 which can be made of these methods is that 

 thermal and mechanical treatments carried 

 out on sheets --^100 m thick may give results 

 which are not tj^pical of bulk material. To 

 overcome this difficulty and to extend the 

 existing thin-foil techniques to hard mate- 

 rials such as alloy steels, which cannot con- 

 veniently be made into sheets of the required 

 thickness, a technique of removing metal 

 uniformly and rapidly from a specimen ~1 

 mm. thick has been developed by Kelly and 

 Nutting (30). This consists in machining the 

 specimen electrolytically with a jet of acid. 

 A glass jet ~1 mm. in dia., connected to a 

 copper-tube cathode, is moved horizontally 

 backward and forward at constant velocity 

 30 times a minute, while the specimen, 

 mounted perpendicular to the jet at a dis- 

 tance of 1-2 mm., is moved vertically up and 

 down once every 6 min., also at constant 

 velocity. The cams imparting these motions 

 have a 1-in. throw, so that a square area on 



the specimen of 1-in. side is covered by the 

 jet. The accuracy of the machining is better 

 than 2% of the metal removed. Using a 

 hydrochloric acid electrolyte at 2 amp. and 

 50 V on a steel specimen, the rate of metal 

 removal is -^250 ju/amp./hr. Since it is pos- 

 sible to eliminate the effects of mechanical 

 cutting and grinding by electromachining, 

 without destroying or modifying the struc- 

 ture of the interior of a metal, thin foils can 

 be prepared from as large a specimen as re- 

 quired. In this way foils of 1 % carbon steel 

 and 20 % nickel, 0.8 % carbon steel were pro- 

 duced from material 0.75 mm. thick, elec- 

 tromachined to 75 /x and then electropolished 

 by the Bollmann technique. 



REFERENCES 



1. Pashley, D. W., Advances in Physics, 5, 173 



(1956). 



2. PiTSCH, W., Phil, Mag., 4, 577 (1959); see also 



./. Inst. Metals, 87, 444 (1958-59). 



3. Trillat, J. J. AND Takahashi, N., Compt. 



rend., 235, 1306 (1952). 



4. Takahashi, N. and Trillat, J. J., ibid., 237, 



1246 (1953). 



5. Takahashi, N. and Mihama, K., Acta Met., 5, 



159 (1957). 



6. Takahashi, N. and Ashinuma, K., /. Inst. 



Metals, 87, 19 (1958-59). 



7. SuiTO, E. AND Uyeda, N., "Proceedings of the 



Third International Conference on Electron 

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8. Weisenberger, E., Z. wiss. Mikroskop., 62, 



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9. Weil, R. and Read, H. J., J. Appl. Physics, 21, 



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10. Reimer, L., Z. Metallkunde, 47, 631 (1956). 



11. Takahashi, N. and Kazato, K., Compt. rend., 



243, 1408 (1956). 



12. Takahashi, N. and Ashinuma, K., ibid., 246, 



3430 (1958). 



13. HiRSCH, P. B., Kelly, A. and Menter, J. W., 



"Proceedings of the Third International 

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 (London, 1954). 



14. Haanstra, H. B., Philivs Tech. Rev., 17, 178 



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15. Reimer, L., T. Metallkunde, 50, 37 (1959). 



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M. J., Phil. Mag., 1, 677 (1956). 



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186 



