For determining the length of the wire in the spool without unwinding it. ammeter, voltmeter and vernier caliper are required. To determine the length of the wire. it is necessary to assemble a chain from a coil and a battery. Using an ammeter and voltmeter, we measure the current and voltage, respectively. Knowing the dependence R = U / I, we find the resistance of the coil. Then, measuring the diameter of the wire with a caliper, we find the cross-sectional area of the wire according to the formula: S = nd ^ 2/4. Then we find the length of the wire using the formula: l = RS / p.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.