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This paper reports experimental findings on the Dirac (Pi) and Pauli {F~)form factors of the proton. The form factors have been obtained by using the Rosenbluth formula and the method of intersecting ellipses in analyzing the elastic electron-proton scattering cross sections. A range of energies covering the interval 200-1000 Mev for the incident electrons is explored. Scattering angles vary from 35' to 145'. Values as high as q'-31 f ' (q= energy-momentum transfer) are investigated, but form factors can be reliably determined only up to about q2= 25 f '. Splitting of the form factors is confirmed. The newly measured data are in good agreement with earlier Stanford data on the form factors and also with the predictions of a recent theoretical model of the proton. Consistency in determining the values of the form factors at different energies and angles gives support to the techniques of quantum electrodynamics up to q'-25 f '. At the extreme conditions of this experiment (975 Mev, 145') the behavior of the form factors may be exhibiting some anomaly.

We have recently put into operation a new large double-focusing magnetic spectrometer capable of analyzing electrons or other singly-charged particles up to a momentum value of 1000 Mev/c. This spectrometer is of the 180° type previously used in this laboratory and has a mean radius of curvature of 72 in. We have employed this spectrometer in electron-scattering work on the proton between incident electron energies of 650 Mev and 900 Mev and between scattering angles of 45° and 145°. A typical value of the solid angle employed in detecting electrons is 5.6xlO" 3 steradian. The higher energies (>650 Mev) have been realized by virtue of the recent extension in the length of the Stanford linear accelerator.The 72-in. spectrometer forms part of a twomagnet system; the second part is the 36-in. spectrometer described previously. 1 Figure 1 is a schematic drawing of the two spectrometers in a position in which they are 120° apart. Both spectrometers are arranged so that they can be rotated independently about a common scattering center. We have taken data simultaneously with both spectrometers, usually employing the 36-in. magnet at large angles, e.g., up to 145°. The 36-in. spectrometer can handle scattered electrons only up to 500 Mev/c without excessive deterioration of focusing and we have used the 36-in. spectrometer in these experiments only in the very safe region below 370 Mev/c. Such a procedure limits, and has limited in the past, the ability of this spectrometer in obtaining scattering data at high energies and small angles, 1 ' 2 i.e., in those circumstances where the energy of the scattered electron is high.We have begun to carry out a series of experiments on the proton at various energies and angles with these spectrometers. In both instruments we used Cerenkov counters as detectors. In. Fig. 2(a) we show a typical electron-scattering

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Electron-scattering studies of the proton obtained in the last few years have been summarized recently. 1 The measurements showed that the proton form factors (F lf F 2 ) were less than unity, implying a finite structure, and lay in a region in which they were approximately equal to each other at momentum transfers (q) as high as # 2 = 9.3 in units of squared inverse fermis. At this value of the momentum transfer the measured ratio was F 1 /F 2 = 1.23 ± 0.20. 2 The experiments were confined to angles larger than 60° at the highest energies then obtainable (650 Mev) because of the limitation imposed by the energy-handling ability of the 36-in. spectrometer. It was therefore not possible to solve for F 1 and F 2 separately at values of q 2 ^9.3. Several independent experiments 2 * 3 indicated that the F ± values were slightly greater than the F 2 values at the same momentum transfer, but for simplicity and ease of calculation, in the past, the ratio of form factors was usually taken to be unity.We have now succeeded in splitting apart the two proton form factors. Because of the great interest in the proton form factors and because our data appear to be internally consistent, we wish to present in this paper some conclusions drawn from the experimental results given in the accompanying paper. 4 Our procedure has been to solve for the separate form factors (F 19 F 2 ) at conditions lying between 7.7 ^ q 2 ^ 25 by choosing a pair of experimentally measured cross sections at the same value of q 2 but at different correlated values of energy and angle. We have used the method of intersecting ellipses 5 to find the form factors. Table I shows the values selected and the form factors found by combining the results. In a few cases, indicated by asterisks, we have used older data and combined the older values with the newly-measured cross section at the same value of q 2 . In two cases (866 Mev, 75°; 675 Table I. Form factors F t and F 2 . Q 2 (f-2 ) 7.70 9.16 11.50 14.06 16.97 18.03 21.24 E t (Mev) 800 700 800 900 866 900 900 0 X (deg) 45° 60° 60° 60° 75° 75° 90° (da/da) t (cmVsr) 1.