Questions to be admitted for doing laboratory work and its defending

1. What measuring devices do you know?

2. What is permissible error limit of electric measuring instruments?

3. What metrological characteristics of measuring devices do you know?

4. Give the definition of accuracy class of measuring devices.

5. What is sensitiveness and scale factor of measuring device?

6. How can absolute and fractional errors of measurements be calculated?

7. What are absolute and fractional errors of current and voltage measurements determined?

8. Does absolute error of current and voltage measurement depend on the value of the measured quantities?

9. How are ammeter and voltmeter measurements ranges chosen?

10. Draw the scheme of the experimental electrical circuit.

11. Deduce the formula for the calculation of unknown resistance R_x.

12. Deduce the formula for the calculation of absolute error of unknown resistance R_x.

13. How can the measurement range of ammeter and voltmeter be changed?

14. Deduce the formula for the shunt resistance calculation.

15. Deduce the formula for the voltmeter additional resistance calculation

LABORATORY WORK

Determination of wire specific resistance

The purpose of the work. To study the method of wire specific resistance determination

Devices and accessories Installation for wire specific resistance determination by ammeter-voltmeter method

Task 1. Wire resistance determination

1.1. Set wire length l₁ (set by a teacher) using moving contact.

1.2. Switch on installation. Button «Bridge» must be pushed. Switch button is pushed when scheme of figure 4.1. is used. And switch button is not pushed when scheme of figure 4.2. is used.

1.3. Write down ammeter and voltmeter readings. Enter the measured values of current and voltage into table 1.

Table 1

Wire length, m	Exp. No	U, V	I, A	Rі, Ω	Rіave, Ω	Rі= Rіave – R0	R, %	ΔR, Ω
l1 =	1
	2
	3
l2 =	1
	2
	3
l3 =	1
	2
	3
l0 =

1.4. Change two times wire length and repeat measurements according to item 1.3.

1.5. Determine wire resistance R₀ setting length l₀ = 0 and repeat measurement according item 3. Enter the results into table 1.

1.6. Determine resistance R_i for each pair of measured U and I according to the formula:

R_і = or R_і = .

Calculate the mean value of resistance R_{i ave} for each wire length l.

1.7. Determine the fractional and absolute ΔR errors of resistance measurement. Enter the results into table 1.

1.8. Determine wire resistance, taking into account wire resistance R₀:

R_i = R_{i ave} – R₀.

Enter the results into table 1.

Task 2. Wire specific resistance determination.

2.1. Measure wire diameter d by micrometer. Enter the results into table 2.

Table 2

Exр. No	і, Ωm	ave, Ωm	Ωm	Ωm	Δ, Ωm	, %	d, m
1
2
3

2.2. Determine wire specific resistance at different values of l_i according to the formula:

_i = .

Enter the results into table 2.

2.3. Calculate arithmetic mean value ρ_ave according to the formula:

where n = 3 is amount of calculations.

2.4. Calculate deviation from mean value .

2.5. Calculate root-mean-square deviation according to the formula:

2.6. Determine the absolute error of wire specific resistance measurements according to the formula:

 = t_St · .

where the Student’s coefficient is t_St = 4,30 (n = 3 and  = 0,95)

2.7. Calculate fractional error according to the formula

2.8. Write down the result in a form:

= Ωm.

Conclusions

The data of laboratory work fulfillment

Pass mark Signature

Mark of laboratory work defending Signature

questions to be admitted for doing laboratory work and its defending

1. What factors does specific wire resistance depend on?

2. What factors have no influence on specific resistance value?

3. How can wire specific resistance be obtained?

4. Write down formula for wire specific resistance determination.

5. What is measurement unit of wire specific resistance?

6. What does temperature resistance coefficient equal?

7. Write down formula for dependence of specific resistance on temperature.

8. What is specific conductivity?

9. Write down formula for dependence of specific conductivity on temperature.

10. What are reasons of systematic methodical error of wire resistance measurement? How is systematic methodical error avoided?

11. Deduce formula for determination of fractional error of specific resistance measurement.

12. How is wire resistance determined? How is branch wire resistance taken into account

LABORATORY WORK