Photocell for h-determine, with housing
06778.00
PHYWE Systeme GmbH & Co. KG
Robert-Bosch-Breite 10
D-37079 Göttingen
Phone
+49 (0) 551 604-0
Fax
+49 (0) 551 604-107
E-mail
info@phywe.de
Internet
www.phywe.de
Operating Instructions
Fig. 1: Photocell for h-determine, with housing 06778.00.
1
SAFETY PRECAUTIONS
The upper part of the casing, on which the circuit diagram is
printed, can be removed after loosening the lateral screw.
The photocell is placed in a tube with two apertures. The
bridge between the orifices avoids direct irradiation of the
central anode, thus avoiding disturbances due to
photoemission from the anode.
• Carefully read these operating instructions completly
before operating this instrument. This is necessary to avoid
damage to it, as well as for user-safety.
• Do not start up this instrument should there be visible signs
of damage to it.
Should it become necessary to replace a photocell, please
send the complete unit to our service department.
• Only use the instrument for the purpose for which it was
designed.
Heating out of the anode, which was necessary with the
formerly used potassium photocells, is no longer necessary
with the new PbS cells. This makes experimenting much
easier and damage to the tube is practically impossible.
2 PURPOSE AND DESCRIPTION
The photocell 06778.00 is used to determine Planck’s
constant h, based on the photoelectric effect. The unit is
equipped with a photocell which uses a PbS-photocathode.
Photoelectric voltages provided by the photocell when it is
irradiated with light of known wavelengths is measured
directly by means of a voltmeter with extremely high
resistance (Ri ≥ 1013 Ω).
3 MEASUREMENT PRINCIPLE
If light impinges on the photocathode, photoelectrons are
emitted due to the exterior photoelectric effect, under the
condition that the energy of the light quanta is higher than
that required for electrons to exit the photocathode (work
function Wa). The kinetic energy of photoelectrons Wk
increases with the energy h·f of the light quanta:
The following experiments can be carried out:
– Demonstration of the photoelectric effect
– Determination of Planck’s constant h
– Determination of the work function and of the cut-off
wavelength of the photocathode
Wk = h·f - Wa.
The photoelectrons reach the anode and load the latter
negatively. The difference of potential increases up to a limit
UG, which is reached when the complete kinetic energy Wk of
the photoelectrons is required to overcome the difference of
potential UG which has built up:
The photocell is placed in a casing which shields it against
disturbing fields. The lateral light entry is fitted with a tube
onto which interference filters may be set. The orifice can be
closed by means of a sliding shutter; „open“ and „closed“
conditions are indicated by graphic symbols.
Wk = e·UG.
(e = electron charge 1.602 10-19 A·s).
06778.00/1106
(1)
1
(2)
Fig. 2: Measurement set-up to determine h with photocell 06778.00.
From equations (1) and (2) one obtains:
e·UG = h·f - Wa.
will occur. The precedent measurement amplifier model with
the order No. 11761.93, which is present in many didactic
equipment kits, is also adequate. The operating modus
„electrometer“ and the amplification factor 1 must be set on
the measurement amplifier. Practically any available
analogue or digital multimeter can be connected to the output
of the measurement amplifier.
(3)
If UG is measured at least for two wavelengths, the two
unknown factors h and Wa can be determined from equation
(3). Normally, however, more than two measurements are
made and UG is plotted as a function of f. After (3) is modified
to
UG =
h⋅ f
e
−
Wa
e
Carrying out a measurement:
– The amplifier input is shorted by pressing button „0“ when
the light entry of the photocell is closed. Whilst the button
„0“ is depressed, the read-out on the measurement
amplifier is set to zero by means of the „0“ setting knob.
– Open the sliding shutter and record the indicated voltage
UG.
– Close the sliding shutter, exchange the filter and repeat
until the measurements have been carried out with all
filters.
(4)
t becomes apparent that this is a straight line with slope h/e.
The work function may be assessed from the negative
ordinate section. However, for PbS cathods, this value does
not have the significance of a physical constant, as it is
significantly influenced by the manufacturing technology of
the photocathode. Once h has been determined, Wa can be
calculated from the intersection of the compensating straight
line with the abscissa, the so-called limit frequency fmin (the
long wave limit wavelength λmax = c/fmin is more usual), for
which UG = 0. In this case, Wa = h·fmin.
The measurement values are then plotted as shown in fig. 3
as an example. Planck’s constant h is determined from the
slope of the straight line, using equation (4).
For the measurement example shown in fig. 3, one obtains
the value h = 6.05·10-34 Ws. Comparing this value with the
value given in literature, h = 6.625·10-34 Ws, the error is
found to be -8.7%.
From the intersection of the straight line with x axis at
fmin = 3.3·1014 Hz, one finds a limit wavelength of
λmax = 908 nm for the present example.
4 HANDLING
The experimental set-up is shown in fig. 2 and is carried out
using the parts mentioned in the list of accessories (cf.
section 5). Instead of the illustrated set-up, the unit can also
be set up on an optical bench by means of the provided
threaded rod which can be screwed into the base plate.
4.1 Indications concerning measurement precision
The determination of Planck’s constant using the
photoelectric effect is not as precise as with other procedures
(e.g. the short wave limit of the X-ray retardation spectrum).
The errors are partly due to unavoidable photoemission from
the anode as well as to disturbing effects in the photocells.
With our photocells, systematic deviations of h from literature
values are within ± 20%. The scattering degree of repeated
measurements with the same cell is low.
Due to historical reasons, this method to determine h should
be the one used preferentially during classes.
The spectral lamp which is connected to the ballast coil
should be switched on about 15 minutes before the first
measurement is carried out. To carry out measurements, it is
shifted near the light entrance orifice (distance approximately
2 cm), an interference filter being pushed over the tube. The
BNC socket is connected by a shielded cable to the input of
the measurement amplifier. It is absolutely necessary to use
either this measurement amplifier, or another one with an
input resistance Ri ≥ 1013 Ω; otherwise, measurement errors
06778.00/1106
2
2
1,5
UG
___
V
1
0,5
0
0
1
2
3
4
5
6
.
7
-14 8
f/10f-3410
Hz
Hz 9
Fig. 3: Measurement example.
5
LIST OF ACCESSORIES
(cf. Fig. 2)
Photocell to determine h
Mounting plate, 210 mm x 160 mm
Experimental lamp 6
Spectral lamp Hg100, pico 9
Ballast coil for spectral lamps
Measurement amplifier, universal
Voltmeter 0.3...300 VDC, 10...300 VAC
Interference filters, set of 3
Interference filters, set of 2
Shielded BNC cable, l = 750 mm
Connecting cable 25 cm, red
Connecting cable 25 cm, blue
7 WASTE DISPOSAL
The packaging consists predominately of environmental
compatible materials that can be passed on for disposal by
the local recycling service.
06778.00
13002.00
11615.05
08120.14
13662.93
13626.93
07035.00
08461.00
08463.00
07542.11
07360.01
07360.04
Should you no longer require this product, do
not dispose of it with the household refuse.
Please return it to the address below for proper
waste disposal.
PHYWE Systeme GmbH & Co. KG
Abteilung Kundendienst
Robert-Bosch-Breite 10
D-37079 Göttingen
Phone +49 (0) 551 604-274
Fax
+49 (0) 551 604-246
6 GUARANTEE
We guarantee the instrument supplied by us for a period of
24 months within the EU, or for 12 months outside of the EU.
This guarantee does not cover natural wear nor damage
resulting from improper handling.
The manufacturer can only be held responsible for the
function and technical safety characteristics of the
instrument, when maintenance, repairs and changes to the
instrument are only carried out by the manufacturer or by
personnel who have been explicitly authorized by him to do
so.
06778.00/1106
3