A major concern for a coin collector is whether the coins you have bought are genuine. One simple test – which works for many modern coins, is the magnetic slide test.
A brief explanation - unlike iron and nickel, copper and silver are not attracted to magnets. However, both silver and copper have a property called ‘diamagnetism’ – in plain language, these metals are repelled by a magnet. This effect is stronger in case of silver and can be noticed with naked eye, especially in case of silver if the magnet is powerful enough. Magnets made of neodymium, a rare earth metal, are powerful enough to display this effect.
You need a number of bar magnets, which have been stuck together to form a sheet. This sheet, or slide now needs to be tilted at an angle of 45 degrees. A coin with no magnetic properties, such as a 5 rupee or a 10 rupee modern coin will slide down such a ramp very quickly whereas a silver coin will slide slowly, as if facing a resistance. You can view the test here
This test works better on modern, machined coins. For example, the British Indian silver coins from 1835-1938 – which are 91.7% silver, can be tested using this method. Similarly, silver 5 kori coins of Kutch (93.7% silver), pre-1964 US coins (90% silver) and milled coins of Hyderbad and Udaipur can be tested using this technique. You can see here this test being done on a range of coins. So if you have a known genuine specimen of any of these coins, you can compare its behaviour on a magnetic slide against that of the test coin. If they differ significantly, you have reason to be suspicious.
However, this test doesn’t work on all coins. For example, the silver rupees from 1940-45 – which are 50% silver, cannot be tested using this technique. Similarly, hand struck silver coins of Indian princely states and older eras often have impurities in the metal which reduces their sensitivity to this test.