  We analyze changes in the thermodynamic properties of a spin system when it
passes from the classical two-dimensional Ising model to the spin glass model,
where spin-spin interactions are random in their values and signs. Formally,
the transition reduces to a gradual change in the amplitude of the
multiplicative noise (distributed uniformly with a mean equal to one)
superimposed over the initial Ising matrix of interacting spins. Considering
the noise, we obtain analytical expressions that are valid for lattices of
finite sizes. We compare our results with the results of computer simulations
performed for square $N=L{\times}L$ lattices with linear dimensions $L =
50{\div}1000$. We find experimentally the dependencies of the critical values
(the critical temperature, the internal energy, entropy and the specific heat)
as well as the dependencies of the energy of the ground state and its
magnetization on the amplitude of the noise. We show that when the variance of
the noise reaches one, there is a jump of the ground state from the fully
correlated state to an uncorrelated state and its magnetization jumps from 1 to
0. In the same time, a phase transition that is present at a lower level of the
noise disappears.
