electron charge/mass experiment, lab-made
Dateca. 1920-1940
Inventory NumberRS0727
ClassificationPhysics Experiment
Subject
Maker
Department of Physics, Harvard University
?
founded 1884
Cultural Region
Place of Origin
Dimensions25 x 61 x 46 cm (9 13/16 x 24 x 18 1/8 in.)
Material
DescriptionInstrumentation mounted on a rectangular wooden base with cylindrical open wooden frame in the middle.
top of cylindrical frame contails a coil, and there is a second coil of the same size at the base of the cylindrical frame. Mounted between the two coils is a vacuum tube with tapered cylindrical necks and tapered pointed tip. There are several binding posts on the wooden base, as well as a large dial knob on a wooden block.
There is some mercury inside the vacuum tube.
top of cylindrical frame contails a coil, and there is a second coil of the same size at the base of the cylindrical frame. Mounted between the two coils is a vacuum tube with tapered cylindrical necks and tapered pointed tip. There are several binding posts on the wooden base, as well as a large dial knob on a wooden block.
There is some mercury inside the vacuum tube.
Signedunsigned
Inscribedwritten in pen on base: e/m;
label at end of tube: PHYS 192;
label: Coils: # of turns: 36.0 / #14 copper wire / mean radius 15.35 cm / mean seperation 16.0 cm / H of earth ~ 0.5th gauss / Fip ~71° } in Lab
FunctionThis is a physics experiment, for teaching purposes, that demonstrates that cathode rays are composed of particles (electrons) that have a definite mass and electric charge. The experiment does this by showing that the ratio of charge to mass (q/m) is constant for all cathode rays.
A relatively constant magnetic field is produced at the center of two coils (Helmholtz coils), where a vacuum tube is placed. Inside the vacuum tube, electrons circulate between a cathode and an anode, accelerated by a voltage. Due to the presence of the magnetic field, the path of the electrons will be helical (making circles around the direction of the magnetic field).
In this experiment, one controls the magnitude of the magnetic field and the voltage between cathode and anode. From these values, and measuring the radius of the circles that the electrons make, one can determine q/m. See below for details:
Particles that are electrically charged are deflected by magnetic fields in proportion to their electric charge. The magnitude of the magnetic force acting on a particle is a = qvB/m, where q=electric charge; v=particle speed; B=magnetic field; m=particle mass.
The force always acts in a direction perpendicular to the direction of the moving particles, making them move in a circular path, that is, accelerated according to the formula for centripetal acceleration a=v^2/r, with r the radius of the circle.
The speed of the particles is proportional to the voltage due to the kinetic energy relationship qV=mv^2/2, where V is the voltage.
From these equations: e/m = 2V/(rB)^2.
The determination of the radius of the electron's path depends on the vacuum tube that is used for the experiment. In some tubes, some tubes are arranged for visual measurements, while in others it is done by the specific location of the anode and measurement of the total current circulating through the tube.
A relatively constant magnetic field is produced at the center of two coils (Helmholtz coils), where a vacuum tube is placed. Inside the vacuum tube, electrons circulate between a cathode and an anode, accelerated by a voltage. Due to the presence of the magnetic field, the path of the electrons will be helical (making circles around the direction of the magnetic field).
In this experiment, one controls the magnitude of the magnetic field and the voltage between cathode and anode. From these values, and measuring the radius of the circles that the electrons make, one can determine q/m. See below for details:
Particles that are electrically charged are deflected by magnetic fields in proportion to their electric charge. The magnitude of the magnetic force acting on a particle is a = qvB/m, where q=electric charge; v=particle speed; B=magnetic field; m=particle mass.
The force always acts in a direction perpendicular to the direction of the moving particles, making them move in a circular path, that is, accelerated according to the formula for centripetal acceleration a=v^2/r, with r the radius of the circle.
The speed of the particles is proportional to the voltage due to the kinetic energy relationship qV=mv^2/2, where V is the voltage.
From these equations: e/m = 2V/(rB)^2.
The determination of the radius of the electron's path depends on the vacuum tube that is used for the experiment. In some tubes, some tubes are arranged for visual measurements, while in others it is done by the specific location of the anode and measurement of the total current circulating through the tube.
Historical AttributesUsed for instruction in the course PHYS 192 at the Department of Physics, Harvard University.
Damon Engineering, Inc.
Date: 1962-1972
Object number: 2006-1-0082
Department of Physics, Harvard University
Date: 1890-1930
Accessories: extra set of coils without multi-tap windings (0156d, e); extra magnet; square wooden case
Object number: 0156a
Department of Physics, Harvard University
Date: 1890-1930
Accessories: extra set of coils without multi-tap windings (0156d, e); extra magnet; square wooden case
Object number: 0156b
Department of Physics, Harvard University
Date: 1900-1920
Object number: 0144a,b
Department of Physics, Harvard University
Date: mid 20th-late 20th Century
Accessories: variety of spare parts cataloged as 2004-1-0408c-g
Object number: 2004-1-0408a
Damon Engineering, Inc.
Date: 1962-1972
Object number: 2006-1-0102
Damon Engineering, Inc.
Date: 1962-1972
Object number: 2006-1-0101
Department of Physics, Harvard University
Date: late 20th Century
Object number: 2004-1-0408c
