Mucronate Electrode with Micrometer

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PHYSICS: C. BAR US

MUCRONATE ELECTRODE WITH MICROMETER* By CARL BARUS DEPARTMINT oF PHYsics, BROWN UNIVZRSITY Communicated May 10, 1927

1. Apparatus and Data.-The adjustment is shown in figure 1 where the electrode E' is attached to the 1/4 inch brass tube a, partially stopped at c by a perforated cylinder. The tube a contains the micrometer screw b, the end of which is tipped by the needle n. Hence by turning b, the point n may be moved from within E'c, and made to project by any small amount beyond it. The zero of this apparatus is arbitrarily found at the position of the micrometer screw (reading y), at which pressure s suddenly begins to appear at E, measured by the U-gauge beyond U. Before this sparklets (sputtering) continually jump across from E' to E and s = 0. The first stable s (for y = 0) is often casual, the fringe displacement s appearing and vanishing alternately with much interferometer turmoil. Immediately thereafter (Figs. 2, 3, 4), y increasing, s is definite and near the maximum. Over half the efficiency is usually lost after the point projection y = 1 mm. The relatively low s values in the earlier work indicated some maladjustment. The electrodes E and E' were, therefore, freshly turned, polished and adjusted more fully in parallel. The effect of this is astonishing as shown in the graphs, figures 2, 3. Both graphs drop from an actual cusp for the initial data were y

=

0.02

s = 0

0.025 560

0.03 520

0.04 cm. 480

e

3etc

At y = 0.025 the sparking is so imminent that it is difficult to catch the fringes between sparks. At y = 0.02, s = 0 and, therefore, the point of the needle is just within the effective surface of the disc electrode; so that the cusp appears within a tenth of a millimeter from the effective limit. Had it been possible to approach this closer, there is no doubt that a higher order of s-values would have been obtained. The field for x = 2 cm. may be estimated as 10 kv./cm. ensuring the stream of ions just before sparking. The corresponding phenomena for other spark gap (x) values are now to be treated. In figure 4, x = 1 cm. while y increases from 0 to nearly 0.5 cm. At this small distance sparks readily jump across and they pass between parts of the electrodes rather than from the salient needle point. At times sparking may suddenly cease whereupon high pressure (s) at once takes its place. Thus there is no doubt that this graph is cuspidal, * Advance note from a Report to the Carnegie Institute of Washington. Cf. Science, 1927, pp. 448-50, vol. lxv.

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though it could not be tested. The s-drop with increasing y is naturally fast, and s = 0 would soon occur since x-y is small. The case for x = 4 cm., figure 5, differs from curve a, as in the former the cusp seems to be actually rounded. The s of the crest, moreover, is less than was expected (cf. Figs. 2, 3). When the needle retreats into the effective confines of the E' electrode, the latter begins to sputter. At smaller distances sparks may pass across. The pressure s drops to zero very nearly. This sputtering following high s values when y is small is very interesting, since it recalls the behavior of the sensitive flame in acoustics. Here also a uniform linear column breaks down into an oscillating or turbulent condition. If in sputtering the ionized winds are alternately positive and negative they would produce no pressure at E.

2

3

..

-_

.4

f

1w 1 X S o .SL 1X__ 4-~~~~s?

2~~~~*1XW o C s os vI c

0

50

/00

/S0

2X

250

300

350

40

40

550

2

Alternatively one may simply assert that the field insulation breaks down when the electrical pressure or field potential energy, F2/8ir, has reached a certain value. The energy which drives the air current is then converted into the heat of the minute sparks and the electric wind ceases. In the absence of sputtering, the ions are taken from E' to E by air convection; hence the pressure s. On the other hand when sputtering occurs and s = 0, the electric current phenomenon is akin to the conduction of electricity in electrolytes though with far more tempestuous collision of ions.

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VOrL. 13, 1927

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A series of experiments in which the actual saliency y' of the needle from the plane of the electrode E' was measured (in the preceding graphs, Figs. 2, 3 4, the y = 0 refers to the first occurrence of sputtering) gave the following data: x =

2 cm.; y' s

= =

0.03 0

0.11 0.27 0.19 0.07 0.05 0.06 140 320 190 480 570 0

cm.

Thus sputtering begins when the needle protrudes half a millimeter from this electrode. Endeavors made to sharpen the rather fine needle brought no consistent results. A needle placed in the U-tube electrode E and removed from the other, E', produced only a just perceptible negative pressure. 2. Miscellaneous Results.-Some relevant measurement of the potential variation of the machine is desirable to accompany the preceding results. These data were obtained with a simple electrometer in which the indications were given by the deflections of a light horizontal flexure needle of aluminum. It was not thought necessary to standardize the apparatus as the deflections suffice the present purposes. One pole of the Winshurst machine was put to earth and the other joined to the electrometer. An example of the results is given in figure 6, in which the potential is rated in arbitrary scale parts for different widths x of mucronate spark gap. The graph V shows the free potential with the machine making 6 rotations per sec. Vr is the residual potential retained after the machine comes to rest (rlt = 0) without being discharged. This is the threshold potential and the electric wind of the preceding paragraphs does not blow (s = 0), until this potential is exceeded. One may note in particular that V is constant after x = 1.5 cm. for the mucronate electrode used. For x < 1.5 cm. the V and V, values are coincident and fall off very rapidly. If we write F = Vlx, treating the axial field, F, as uniform, the graphs F and Fr are obtained. Both curves have their crests at x = 1 cm., which should, therefore, be the strongest field used. In the preceding experiments (Science, loc. cit., s, x graphs), the electric wind crests occurred at x = 2 to 2.5 cm., therefore, in a materially weaker field F. It is to be observed, however, that for x = 1, the electrodes are already so close together that the electric wind must in large measure be radially outward. Hence only a component pressure acts at the electrode E. Not until the value of x has become larger (x = 2) will the wind in the main be axial. It is somewhat surprising that the limiting potential is practically independent of the speed of the machine. Thus far spark gaps x = 1.5, 2.0, 4.0 cm. each at rotations 1.5, 3, 6 rot./sec. the same limiting potential was built up, more gradually, of course, for the slower motions. The endeavor to use the mucronate electrode in case of an induction coil just below sparking, in a given gap, did not succeed. Values of s

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between s = 7 and 15 finally s = 30, could be obtained for x = 3.5 cm.; and there was even then much sputtering. It remained to determine the effect of the speed of rotation (r/sec.) of the given electric machine on the s-values. When x is constant, s increases nearly proportionally to the angular velocity of the plates, 1.5, 3 and 6 rotations of each plate per second being instanced. The lines pass through zero as x increases from 0 to 1 to 2 cm., the optimum spark gap. The rates at which s increases with r/sec. thus steadily increase. After this (x = 2 to 4 cm.) the line merely drops, the rate of s increase with r/sec. remaining about the same. Thus, for instance, if x = 4 cm. 1.5 rotations per second leave s unchanged at zero. The effect of this on the sx graphs is apparent: the crests persist at x 2 cm.; but that the graphs drop as a whole when r/s diminishes from 6, to 3, to 1.5 rot./sec. 3. Velocity of the Winds.-An estimate may perhaps be obtained if we use Bernoulli's equation and put v = 'p%p. In the graphs, figures 2, 3, 4, the s-values at cusps are very commonly s = 300 and they mount to even s = 550. Since the unit of s is about 10-6 atm., these data may at once be taken as pressures in dynes/cm.2 Thus the velocities in the two cases are v = 770 cm./sec. frequently and v = 1000 cm./sec. in very. favorable cases. These are astonishingly large values. In .the small time of xlv where x = 6 cm., there is very little time for the decay of ions. The change of s with x is thus to be associated with a ring-shaped vortex of air, whose axis or line of symmetry is the needle prolonged. Hence the currents near the electrode E, when x is small, must be largely radial and outward as already instanced. The bearing of much of this, on the cathode minimum potential, will be treated later. THE REFLECTION OF ELECTRONS FROM CRYSTALS By CARL ECKiART* CALIFORNIA INSTITUTh OF TZCHNOLOGY

Communicated May 11, 1927

The recent experiments of Davisson and Germerl on the reflection of electrons from a crystal of nickel have shown a strong analogy between this phenomenon and the reflection of X-rays from the same crystal; the analogy is not complete, however, and the essential differences may be summarized in the following two hypotheses. I. A single plane of atoms reflects a very appreciable fraction of the electron wave, whereas the same plane would reflect only an inappreciable part of an X-ray wave.2