# questions on wattmeter

#### chirpy

Joined Nov 20, 2007
3
hey,can anyone tell me the significance of multiplying factor of wattmeter n what wud happen if the coil connections are reversed in it

#### beenthere

Joined Apr 20, 2004
15,819
The device used to be an AC hysteresis motor - hard for reversed terminal connections to have much effect. There are several ways to make such a meter, so the "multiplying factor" needs to be expanded on.

#### The Electrician

Joined Oct 9, 2007
2,785
Are you referring to a wattmeter such as this:
http://cgi.ebay.com/Weston-AC-DC-Wattmeter-Model-310-Certified-in-1967_W0QQitemZ130175848796QQihZ003QQcategoryZ69854QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

or are you referring to a kilowatthour meter, such as the kind that hangs on the outside of your house?

If the first, then the multiplying factor is the number that you must use to multiply the reading the meter gives.

A wattmeter has two inputs, a voltage coil and a current coil. If you reverse either one, the meter will read downward (assuming it was reading upward before you reversed the connection); if you reverse both the meter will still read upward.

#### chirpy

Joined Nov 20, 2007
3
beenthere n electrician,thanks for your replies........
and by reversing coil connections,i meant to ask what if pressure coil is connected in place of current coil and vice-versa...
and what is,actually,the need to multiply the multiplying constant to get the real power!!!!!
can you help me with that

#### The Electrician

Joined Oct 9, 2007
2,785
The current coil is a very low resistance, and the voltage coil is a high resistance. If you connect the current coil to a source of voltage you will very likely cause damage to the wattmeter and, if you are lucky, blow the fuse which protects the voltage source.

This is the same thing that happens when you connect an ammeter across a voltage source by mistake.

If you connect the voltage coil where the current coil should be connected, the resistance of the voltage coil will probably cause the current to the load to be limited by the resistance of the voltage coil. In other words, the load current will not be determined by the characteristics of the load, but by the resistance of the voltage coil. This will upset the operation of the load.

If you look at the picture of the wattmeter in the URL I gave, you will see that there are multiple terminals to connect to the load and source. This is so the wattmeter can measure different amounts of power. Some wattmeters have multiple scales for different full scale readings, but some, like the Yokogawa type, have only one scale. So, to measure different amounts of power when you use the various current coil connections and different voltage coil connections, there is a multiplying factor. There is usually a chart on the meter that shows which multiplying factor to use with the various connections.