Access object properties across several code files

Hi.

I have a problem with the scope of an object in my C# desktop application for Windows. I would like to instantiate an object and have the object's properties available in my code files behind a number of different windows forms. However, I am seeing CS0103 errors saying "the name does not exist in the current context". I had assumed that because all of the code files use the same namespace, that the object's properties would be available but I've obviously gone wrong somewhere.

namespace TDTestSNWF
{


    public partial class Main : Form
    {
        Transducer newTD = new Transducer(-1);
       
        
        public Main()
        {

            InitializeComponent();

            newTD.SetTDType(-1);

        }

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using TDTestSNWF;

namespace TDTestSNWF
{
    public partial class frmSelectTestType : Form
    {

        public frmSelectTestType()
        {
            InitializeComponent();
            
        }

        
        public void UpdateSelectTestTypeForm()
        {
            string strTDType;


            newTD.SetTDTypeDesc("TEST");

            switch (newTD.GetTDType())
            {
                case 1:
                    this.Hide();
                    strTDType = "Non-Smart transducer";
                    break;
                case 2:
                    strTDType = "Smart transducer";
                    break;
                case 3:
                    strTDType = "Non-Smart transducer with zero";
                    break;
                default:
                    strTDType = "Unrecognised transducer type";
                    break;
            }
        }

The error appears in the second code snip where newTD is referenced.

What am I doing wrong?