This provides an alternative to using a conductivity meter. Even though 9-volt batteries are cheaper and safer, their voltage may drop during discharge if they have been heavily used. This would make your readings unreliable. If you want to use 9-volt batteries they need to be fresh. Lead-acid batteries work best because their voltage only drops when they are almost completely discharged. Graphite electrodes are used because electrolysis process is very corrosive in most metals.