User:Darrell Bonn/Notebook/307L Lab book/lab 2 e over m

e/m lab

Darrell and Boleszek

Equipment and Setup

The e/m apparatus set up and use is clearly stated in the Prof. Gould's manual. The procedure for setting up and using the equipment was followed with some care by us here. DMM's were used to measure the voltage on both the heating filament and the accelerating voltage. Current to the magnet coils was read from the output of the power supply.

We did experiment with higher ranges for the filament and settled on 10v. Adjusting the "focus" brings few results, but it does seem to produce a somewhat easier to see electron beam when twisted full clockwise.

The reading of the radius of the electon beam is very innacurrate. We feel that radius readings have an uncertainty of up to 0.5cm. We decide that we will both make separate estimates of the radius in order to reduce systematic error (of each of us having a slightly different way of reading) and also to produce more data points in hopes that their average will be more accurate. Thus there are four radius measurements for each configuration. Other measurement errors seem likely to pale in comparision. However, the DMM's on the voltage are likely accurate to at least .01 volts for the filament and 0.1Volts for the accelerating voltage. The current on the magnet supply reads to 0.1, and can readily be interpolated to 0.025 amp resolution. However, the accuracy of these front panel vernier displays is often questionable on older supplies.

Prior to beginning we notice that there is a 20mA current still flowing in the accelerating voltage power supply when the filament voltage is set to 0. This goes away when the power supply is unplugged from the e/m device. We are unsure where this small current leakage occurs. It seems likely that it is natural to the device. Accelerating voltage was set to 445 when this was measured. During normal operation it drew 40mA.

Day 1 Data

Filament Voltage: 10V

Accelerating Voltage    Magnet Current    Diameter
405                     1.35              

375                     1.0               3.7 3.5 3.8 3.8
375                     1.2               3.8 4.8 4.0 4.5
375                     1.3               3.8 4.2 3.9 4.2
375                     1.4               3.8 4.6 3.9 4.4
375                     1.5               3.8 4.0 3.5 4.1
375                     1.6               3.4 3.2 3.2 3.5

day 2

Used DMM's to read voltage/current on all three power supplies. This significantly increases our precision for the current reading. However, we do note that it tracks well with the front panel display which seems to be pretty accurate.

Again we find that adjusting the focus full clockwise gives best results. We also decide that lowering the filament voltage improves our ability to see where the beam is.

Constant Voltage data

Accelerating Voltage: 360

Magnet I (amps)   Radius (cm)
1.22              3.4 5.2 3.8 5.2 
1.32              3.4 5.1 3.6 5.0 
1.42              3.0 4.7 3.3 4.7 
1.52              2.9 4.5 3.5 4.4 
1.62              2.9 3.9 3.0 4.0 
1.72              2.4 3.5 3.0 3.7 
1.27              3.5 5.2 3.5 5.3 
1.37              3.4 5.0 3.4 4.8 
1.47              3.2 4.8 3.5 4.5 
1.57              3.0 4.3 3.3 4.4 

Constant Current Data

Magnet I: 1.35

Accel V           Radius (cm)
441.2             4.0 6.0 4.0 5.5 
430.5             4.0 5.5 4.0 4.5
420.0             3.9 5.4 3.9 5.3
409.8             3.9 5.3 3.8 5.3
399.9             3.8 5.2 3.8 5.1
390.1             3.7 5.1 3.7 5.0
380.0             3.5 5.0 3.6 4.8
369.8             3.4 4.9 3.5 4.8
360.0             3.3 4.8 3.5 4.8
350.0             3.3 4.8 3.4 4.5