Here are two infinite wire with current I flowing through and I changes with time $I=kt+I_0$. They are $3a$ apart from each other and a square metal loop with length $a$ is placed at $a$ from one of the wire. The illustration is shown above.
It is easy to calculate the magnetic field from the Ampere's law along with the superposition principle.
For the magnetic field induced by a single wire, it is $B=\mu_0 I/2\pi r.$
The corresponding magnetic flux is $\phi=a \int_a^{2 a} \mu_0 I/2\pi r d r =a \mu_0 I \ln2/2\pi $ For two such wires, the total amount is $\phi=2 a \mu_0 I \ln2/2\pi $.
Then the electromotive force is $\varepsilon = -\frac{d \phi}{dt}= -a \mu_0 k \ln2/\pi$
The electrical resistance in AB, BC is $R_1$, and the electrical resistance in AD, DC is $R_2$. $R_1>R_2$.
According to the Ohm's law, the current in ABCD is $a \mu_0 k \ln2/\pi(R_1+R_2)$ The potential difference is $R_1 a \mu_0 k \ln2/\pi(R_1+R_2)$ from ABC, which is different from the potential difference of ADC,$R_2 a \mu_0 k \ln2/\pi(R_1+R_2)$.
So my question is what is the real potential difference between A and C, since a voltmeter must have a value if it is connected to this two points ?