24 Taking the resistivity of platinoid as 3.3 x 10-7 
m, find the resistance of 7.0 m of platinoid wire of average diameter 0.14 cm.
25 The resistance of the ohm is very approximately that of a column of mercury 1.06 m long and of uniform cross-section of one hundredth of a cm2. Find the resistivity of mercury.
26 What length of German silver wire, diameter 0.050 cm, is needed to make a 28 resistor, if the resistivity of German silver is 2.2 x 10-7 m?
27 The maximum allowable resistance for an underwater cable is one hundredth of an ohm per metre. If the resistivity of copper is 1.54 x 10-8 m, find the least diameter of a copper cable that could be used.
28 A block of carbon, 1.0 cm by 2.0 cm by 5.0 cm, has a resistance of 0.015 between its two smaller faces. What is the resistivity of carbon?
29 A uniform strip of eureka (resistivity 5-0 x 10-7 m) has a resistance of 0.80 per metre and is 0.25 cm wide. What is its thickness?
30 A wire of uniform cross-section has a resistance of R . What would be the resistance of a similar wire, made of the same material, but twice as long and of twice the diameter?
31 A wire of uniform cross-section has a resistance of R . If it is drawn to three times the length, but the volume remains constant, what will be its resistance?
32 A simple way to make a resistor of high resistance is to draw a line with a graphite pencil across a sheet of insulating material. One such line has a resistance of 10 M and is 80 cm long. If its width is 1.8 x 10-2 cm, what is its average thickness? (Resistivity of graphite = 6.3 x l0-5 m.)
ANSWERS 24) 1.50 25) 9.4 (9.434) x lO-7m 26) 25m 27) 0-14cm 28) 6.0xl0-5m 29) 0.025 cm 30) 1/2 R 31) 9R 32) 2.8 x 10-4 cm