Introduction:
     Magnetism has been studied for several centuries.  A magnet is something that usually attracts magnetic metals such as iron to itself. Magnets all have two charged poles,  positive and negative.  When two poles with like charges are placed in a way so that their magnetic fields overlap,  they will repel each other.  The poles will attract each other if the carry opposite charges. Magnetic fields can also be described in terms of their effect on electric charges. A moving electron, that carries a charge will change velocity and its direction of travel in the presence of a magnetic field.  The properties of magnetism were observed originally in the form of lodestones(charged rocks that always pointed North when suspended free of obstructions),  and latter in the form of bar magnets and charged metals.  The earth has a magnetic field which explains the movement of lodestones and compasses.
    In this lab, We examined the path of the magnetic field lines,  and studied the properties of magnetism. The main objective was to investigate effect of magnetic fields on electrons.  We observed the behaviors of magnetic fields generated by electric current, as well as those propagated by simple bar magnets.  A cathode ray tube was used to observe the motion of electrons under the influence of a magnetic field,  and then to calculate the charge to mass ratio of the electrons.

Procedure:   To see a formatted version of these experiments click here.

    The magnetic field of a bar magnet was the first field that we studied.  We took two bar magnets and placed them on the overhead projector underneath a transparency.  With the projector on,  iron fillings were placed on the transparency,  showing the magnetic field.  The fillings were attracted to the north and south ends of the magnet, and arranged in between the two poles in arcs.  The two magnets were then pulled away from each other and iron fillings were placed on the transparency again.  This time,  the fillings were attracted to both poles of both magnets, but deflected in between.  This demonstrated that when a magnet is split,  each piece would continue to have a north and south pole.  Next,  we used magnets to also observe this effect.  When the south end of the magnet was placed near a compass,  the arrow points towards it.  This finding supported the previous observation about opposite charges attracting,  as a compass is simply a small magnet.








The magnetic field of the earth is similar to the bar magnet which explains how a compass works.  The magnetic field of earth is three dimensional,  and therefore acts on different areas at different angles.  This is angle is commonly known as the dip angle and is defined as the angle at which the magnetic fields are inclined to the horizontal.  This principle was observed by placing a cow magnet in paraffin oil and iron fillings.  It was then possible to see the magnetic field on a three-dimensional level.  All of the fillings were arranged around the exposed pole sticking out at various angles form the magnet.

Next,  we began to look at magnetic fields generated by flow of charge(electric current.)  Placing four compasses around a wire that was perpendicular to them did this.  When a current was passed through the wire,  from the bottom to the top,  the compass needles turned in a counterclockwise direction.  When the current flow was reversed,  the compasses  moved in the opposite direction(clockwise.)  This demonstrated that the magnetic field wraps around a current in relation to the direction of flow.  We then removed the compasses and replaced them with iron fillings.  The findings of this trial supported the prior evidence that direction of current was responsible for the magnetic deflection.
 The final qualitative analysis of magnetism brought it to the atomic level.  We observed the effect of a magnetic field on electrons.  This observation was done using a Cathode ray tube(*Cathode ray tubes were invented by JJ Thomson.) To learn more bout the history of the Cathode Ray tube click here. A heater coils creates a thermionic emission in which negatively charged electrons are projected at an anode.  Some intercept the anode while others are able to slip through at a high velocity.  The electrons reach the end of the tube,  react with a phosphor and rise to an excited state.  When they return to ground state,  a green light is emitted from the end.
 Several types of magnets were placed next to the CRT in order to observe the effects of magnetic deflection on electrons.  The results of these tests led us to believe that the force on electrons is perpendicular to the magnetic field.  For example,  when the north end of the magnet was placed on top of the CRT,  the electron beam shifted to the left.  The magnetic field was generated using bar magnets and Hemholtz coils of different strengths.  The velocity of the electrons was found to decrease proportionately to the decrease in voltage.  The electrons are therefore,  under the influence of the magnetic field for a longer period of time, causing a greater divergence from the original path.
 Next,  we used the TEL 525 as a quantitative measuring device.  This is another form of cathode ray tube with the addition of a graticle (flat grid).  In the TEL-525,  the electron beam travels along a slanted graticle, which is also a phosphor.  As the electrons react with the phosphor,  a light is emitted along the entire graticle representing the position of the electrons.  Since the distance between each line is known,  it is possible to displace the electrons a measurable distance by adjusting magnetic field and voltage.  The magnetic field was created using two Hemholtz coils with 320 turns each. In order to produce an even magnetic field around the TEL 525,  these two coils were separated from each other at a distance approximately equal to the radius of themselves. The following equation was used to calculate the magnetic field We know that the magnetic force on  an electron traveling with velocity  perpendicular to magnetic field B:
equation 1
 

We recorded the current and voltage required to displace the electron beam +/- 2.4cm.  The current values were then averaged to get a more accurate value.  The data yielded the following numbers.

We were able to use the known energy of the electrons, and the amount of magnetic deflection to calculate the charge to mass ratio of the electrons in coulombs per kilogram. The specific charge of the electrons in the Cathode ray tube was calculated using the following equation dervied asshown below.

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The radius of the electrons motion was calculated based on the beams position on the graticle. Our r-value was determined to be 0.312 based on the equation




This value was then substituted along with the magnetic field values into the charge/mass equation to get the corresponding values.

Conclusions:
The mean value of our charge to mass ratio was compared with the literature value,  in order  to determine the error.  We received a percent error of 40.  Our charge/mass ratios were all in the same area, which indicated a high degree of precision,  our accuracy,  however was not so good.  This is due to the change in strength of magnetic field at different points of the system.  What this means is that the field was probably weaker towards the edge of the coil,  and our calculated B value was for the center were it is the strongest.  For other interpretations of magnetism check out this cool web sites.