Man's physical universe



of their discoverer. We will come back to this important discovery

later. Let us turn to still other observations.

In 1897 /. /. Thomson attempted to measure the speed with which

these cathode particles moved and the ratio of their electric charges to

their masses. In October, 1897, he made

an historic experiment in which he

found that the cathode rays could be

deflected by a magnetic field. With his

apparatus, shown in Fig. 270, he could

measure the deflection of the cathode

rays by an electromagnet when they

passed between oppositely charged

plates. The whole apparatus was placed

between the poles of a powerful electromagnet.

Fig. 269. Vacuum tube to show

the deflection of cathode rays by

With this apparatus Thomson

a magnet. (Courtesy of the Central

Scientific Company.)

was able to show that the ratio of the

charge on each particle to its mass was constant regardless of the

nature of the electrodes or of the gas in

the tube.

It was suspected

that the mass of these particles was very small, but so far neither the

mass nor the charge had been actually measured.

In 1898 and 1899 Thomson measured the value of the charge by the

cloud method originated by C. T. R. Wilson, later improved hy H. A.

Wilson, and still later further improved by Robert Millikan, who

confirmed the results of Thomson.

This method was based on the fact

Fig. 270. Sir. J. J. Thomson's

apparatus by which the velocity and

the ratio of the mass of the electron

to its charge was determined.

C, flat

circular cathode; D, anode pierced

by a central hole; E, brass plug

pierced by a similar hole; A and

B, parallel plates which establish the

electrostatic field; p to p', phosphorescent

spot deflected by a magnetic


that these negatively charged particles

would act as nuclei to condense

droplets of water from moist

air. The size of the drops would be

determined by the rate at which

they would fall. The total quantity

of charge used, divided by the

number of drops produced, gave

the charge per drop.

The ratio of the charge to the

mass, as determined, turned out to

be about 1847 times as great as the

same ratio for the smallest known

particle of charged matter, the hydrogen ion. These cathode particles

must, therefore, either carry a charge much greater than that carried

by the hydrogen ion or have a much smaller mass. If their charge

were the same as that of the hydrogen ion, their mass would be 1/1847

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