Ups Dealers in Adarsh Nagar, Roorkee

We offer Four Probe Method it is one of the standard and most widely used method for the measurement of resistivity. In its useful form, the four probes are collinear. The error due to contact resistance, which is significant in the electrical measurement on semiconductors, is avoided by the use of two extra contacts (probes) between the current contacts. In this arrangement the contact resistance may all be high compare to the sample resistance, but as long as the resistance of the sample and contact resistance's are small compared with the effective resistance of the voltage measuring device (potentiometer, electrometer or electronic voltmeter), the measured value will remain unaffected. Because of pressure contacts, and 2 way motion, the arrangement is specially useful for quick measurement on large samples at room temperature. Features Easy resistivity mapping of large sample 2-way movement with vernier scales (0.01mm) Spring loaded contacts for firm connections Experiment consists of the following: 1. Probes Arrangement, FPA-01 It has four individually spring loaded probes. The probes are collinear and equally spaced. The probes are mounted in a teflon bush, which ensure a good electrical insulation between the probes. A teflon spacer near the tips is also provided to keep the probes at equal distance. The probe arrangement is mounted in a tube, which also provide leads for connections to Constant Current Power Supply and D.C. Microvoltmeter. The tube containing four probes is mounted on a travelling microscope type system, scales and verniers are made of stanless steels with following specification: Horizontial: 20 cm least count 0.001 cm Laterial: 6 cm least count 0.001 cm Vertical: 15 cm least count 0.001 cm The bed is of heavy casting, thoroughly aged and machined, is fitted with leveling screws. A large platform is provided for fixing the sample. 2. Constant Current Source a) Constant Current Source, Model : CCS-01 (for low resistivity samples) The current source is suitable for the resistivity measurement of low to medium resistivity samples such as thin films of metals/ alloys and semiconductors like germanium. Specifications Open Circuit Voltage: 12 V Current Range: 0-20mA, 0-200mA Resolution: 10μA Accuracy: ±0.25% of the reading Display: 3½ digit, 7 segment LED Load Regulation: ±0.1% for 0 to full load Line Regulation: ±0.1% for 10% changes b) Low Current Source, Model : LCS-02 (for high resistivity sample) This current source is especially suitable for the resistivity measurement of polymer ceramics and Si crystals. Specifications Open Circuit Voltage: 15V Current Range: 0-2μA, 0-20μA, 0-200μA & 0-2μA Resolution: inA at 0-2μA range Accuracy: ±0.25% of the reading ±1 digit Display: 3½ digit, 7 segment LCD with autopolarity Load Regulation: ±0.1% for 0 to full load Power: 3 x 9V batteries 3. D.C. Microvoltmeter, DMV-001 The experiment complete in all respects.

We offer Magnetoresistance set-up, it is noticed that the resistance of the sample changes when the magnetic field is turned on. The phenomenon, called magnetoresistance, is due to the fact that the drift velocity of all carriers is not same. With the magnetic field on; the Hall voltage V = Eyt = v X H compensates exactly the Lorentz force for carriers with the average velocity; slower carriers will be over compensated and faster one undercompensated, resulting in trajectories that are not along the applied field. This results in an effective decrease of the mean free path and hence an increase in resistivity. Here the above referred symbols are defines as: v = drift velocity; E = applied electric field; t = thickness of the crystal; H = Magnetic field. The set-up consists of the following: (1) Four Probe arrangement, MRA-01 It consists of 4 collinear, equally spaced (2mm) and individually spring loaded probes mounted on a PCB strip. Two outer probes for supplying the constant current to the sample and two inner probes for measuring the voltage developed across these probes. This eliminates the error due to contact resistance which is particularly serious in semiconductors. A platform is also provided for placing the sample and mounting the Four Probes on It. (2) Sample Ge Crystal (n-type) dimensions: 10x10x0.5mm. (3) Magnetoresistance Set-up, Model DMR-01 This unit consists of a digital millivoltmeter and constant current power supply. The voltage and probe current can be read on the same digital panel meter through a selector switch. (a) Digital Millivoltmeter Intersil 3.5 digit single chip ICL 7107 have been used. Since the use of internal reference causes the degradation in performance due to internal heating an external reference have been used. Digital voltmeter is much more convenient to use, because the input voltage of either polarity can be measured. Specifications Range : 0-200mV (100μV minimum) Accuracy : +-0.1% of reading +-1 digit (b) Constant Current Power Supply This power supply, specially designed for Hall Probe, provides 100% protection against crystal burn-out due to excessive current. The supply is a highly regulated and practically ripple free dc source. Specifications Current : 0-20mA Resolution : 10micro ampere Accuracy : +-0.2% of the reading +-1 digit Load regulated : 0.03% for 0 to full load Line regulation : 0.05% for 10% variation (4) Electromagnet, EMU-50V (5) Constant Current Power Supply, DPS-50 (6) Digital Gaussmeter, DGM-102/DGM-202 The Set-up complete in all respect.

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