You need an amp meter. You are measuring voltage which will not help as it looks to me like you have all your components hooked up. This is like having a pneumatic (air under pressure) system where by using an air ratchet some pressure is lost by doing work, ie making the ratchet spin. Measuring before a leak as big as a heater will cause some back-up pressure or voltage, but much will be used by the heat element.
A inductive meter is available at your local home improvement center... Lowes or Home Depot. If you can not access these then buy it online which is always cheaper anyway.