$99$ identical balls lie on a table. $50$ balls are made of copper, and $49$ balls are made of zinc. The assistant numbered the balls. Once spectrometer test is applied to $2$ balls and allows to determine whether they are made of the same metal or not. However, the results of the test can be obtained only the next day. What minimum number of tests is required to determine the material of each ball if all the tests should be performed today?

[i]Proposed by N. Vlasova, S. Berlov[/i]
1. **Initial Setup and Problem Understanding**:
   - We have 99 identical balls: 50 made of copper and 49 made of zinc.
   - We can test pairs of balls to determine if they are made of the same metal.
   - The goal is to determine the material of each ball with the minimum number of tests.

2. **Upper Bound on Number of Tests**:
   - Consider picking one ball and testing it with each of the other 98 balls.
   - This will result in 98 tests.
   - After these tests, we will have two sets: one set of balls that are the same as the chosen ball and another set that is different.
   - Since we know there are 50 copper balls, the larger set will be the copper balls, and the smaller set will be the zinc balls.
   - Therefore, 98 tests are sufficient to determine the material of each ball.

3. **Lower Bound on Number of Tests**:
   - We need to show that 97 tests are not sufficient.
   - Consider the balls as vertices of a graph, and each test as an edge between two vertices.
   - If we perform 97 tests, we will have 97 edges in the graph.
   - A graph with 99 vertices and 97 edges is not connected (since a connected graph with 99 vertices must have at least 98 edges).
   - Let \(C_1, C_2, \ldots, C_k\) be the connected components of the graph. Since the graph is not connected, \(k \geq 2\).
   - Each connected component is a tree (since it is acyclic and connected).
   - The number of edges in a tree with \(n\) vertices is \(n-1\).
   - Therefore, the total number of edges in the graph is \(\sum_{i=1}^k (|C_i| - 1) = 99 - k\).
   - Given that we have 97 edges, we have \(99 - k = 97\), so \(k = 2\).
   - Thus, we have two trees \(T_1\) and \(T_2\) with \(|T_1| + |T_2| = 99\).

4. **Coloring and Consistency**:
   - Without loss of generality, assume \(|T_2|\) is even.
   - Label half of the vertices of \(T_2\) as blue (copper) and the other half as green (zinc).
   - Label half (approximately) of the vertices of \(T_1\) in one color and the other half in a different color.
   - Assign Boolean values to the edges based on whether the endpoints are the same color or not.
   - We can have either \(\frac{1}{2}(|T_1| + 1) + \frac{1}{2}|T_2| = 50\) blue vertices or \(\frac{1}{2}(|T_1| - 1) + \frac{1}{2}|T_2| = 49\) blue vertices.
   - Given only the Boolean values of the edges, we cannot determine which set of balls is copper and which is zinc.

5. **Conclusion**:
   - Since 97 tests are not sufficient to determine the material of each ball, the minimum number of tests required is 98.

The final answer is \(\boxed{98}\).