AC mains sonometer

 

In this experiment, constantan wire was clamped at one end and the other hung over a clamped pulley with slotted masses hanging from the end. The wire runs over two prisms that act as knife edge support. Two pole-faced Magnadur magnets from an electric motor kit sit between the prisms and AC is fed into the wire at either end. The wire was 36swg and I used 2 Volts. I started with 100 grams and then went up in increments of 50 grams. The wire oscillates because the magnetic force on it is perpendicular to the current in the wire and dependent on the direction of the current. Hence AC makes it change direction with a frequency of 50Hz. For 100 grams, the prisms were about 30cm apart and that increases as the mass increases. I came up with a measure for mass per unit length of the wire by knowing the density and measuring the radius with a micrometer screw gauge. Using this equation I plotted L-squared against T and was able to use the gradient to get a value for the frequency.