We describe a result coming from an experiment based on an Al-Mg alloy ( \sim 5 \% Mg ) suspended bar hit by an electron beam and operated above and below the termperature of transition from superconducting to normal state of the material .
The amplitude of the bar first longitudinal mode of oscillation , excited by the beam interacting with the bulk , and the energy deposited by the beam in the bar are the quantities measured by the experiment .
These quantities , inserted in the equations describing the mechanism of the mode excitation and complemented by an independent measurement of the specific heat , allow us to determine the linear expansion coefficient \alpha of the material .
We obtain \alpha = [ ( 10.9 \pm 0.4 ) T + ( 1.3 \pm 0.1 ) T ^ { 3 } ] \times 10 ^ { -10 } { K ^ { -1 } } for the normal state of conduction in the temperature interval 0.9 < T < 2 K and \alpha = [ ( -2.45 \pm 0.60 ) + ( -10.68 \pm 1.24 ) T + ( 0.13 \pm 0.01 ) T ^ { 3 } ] \times 10 ^ { -9 } % { K ^ { -1 } } for the superconducting state in the interval 0.3 < T < 0.8 K .