add custom rules in jquery form validate plugin

add custom rules in jquery form validate plugin

How can I add a customized rules after largeThan to call another method to verify that the charge can not be larger than amount. Many thanks!

$("#wu").validate({
event: "keyup",
rules: {
amount: {
required: true,
number: true,
max: 5000,
min: 2

}, //end of amount

charge: {
required: true,
number: true,
max: 200,
//min: 1,
largeThan: //how to define a statement to call the customized method
}, //end of amount

code: {
required: true,
number: true,
minlength: 10
} //end of amount

},

messages: {
amount: {
require: "Require field",
number: "Please enter correct numbers format",
max: "Can not send/receive over 5000 pounds",
min: "Can not send money less than 1 Pound",
},
charge: {
require: "Require field",
number: "lease enter correct numbers format",
max: "please enter correct charge",
// largeThan: "charge can not be large than sending amount",
},
code: {
require: "Require field",
number: "lease enter correct code ",
minlength: "code must be 10 digits",
}

} // end of message
});//end of wu form validate