Ernest Rutherford sought to test Thomson's model.
To accomplish this, he shot a piece of gold foil with a beam of particles.
He expected they would travel straight through, as there was nothing large enough
to deflect the beam in the accepted model of the atom.
He discovered, however, that several of the particles were deflected, and some
even were sent bouncing straight back.
He compared this result to a howitzer shell being reflected back by a piece of paper.
With this result, Rutherford concluded that there must be a center of positive charge,
which he called the nucleus, that was reflecting the particles he shot.