It is called an ohm meter. It is used to check Voltage,amps,ohms. What we call a multimeter today. This looks to be a linesman type. Probably belonged to someone who checked power lines or phone lines on poles. Looks like a nice find. I use a multimeter on the job but it is of the newer type. I have Fluke brand meters. One word of caution, if you decide to try it out find someone you know who does electrical work and have them show you how to use it. If you set it to read ohms and plug it into an outlet it would probably explode the dry cell. You really need to know what settings to have it on before trying it. And being that you don't know much about it I would advise letting an electrician test it first.
David B.